diff --git a/WORKSPACE b/WORKSPACE
index 6ca5c6015a..4404d6b88e 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -68,6 +68,12 @@ load("@build_bazel_rules_nodejs//:index.bzl", "npm_install", "yarn_install")
 
 yarn_install(
     name = "npm",
+    data = [
+        "//internal/npm_install/test:postinstall.js",
+    ],
+    environment = {
+        "SOME_USER_ENV": "yarn_is_great",
+    },
     # The @npm//:node_modules_filegroup generated by manual_build_file_contents
     # is used in the //packages/typescript/test/reference_types_directive:tsconfig_types
     # test. For now we're still supporting node_modules as a filegroup tho this may
@@ -192,6 +198,12 @@ local_repository(
 
 yarn_install(
     name = "fine_grained_deps_yarn",
+    data = [
+        "//internal/npm_install/test:postinstall.js",
+    ],
+    environment = {
+        "SOME_USER_ENV": "yarn_is_great",
+    },
     included_files = [
         "",
         ".js",
@@ -206,6 +218,12 @@ yarn_install(
 
 npm_install(
     name = "fine_grained_deps_npm",
+    data = [
+        "//internal/npm_install/test:postinstall.js",
+    ],
+    environment = {
+        "SOME_USER_ENV": "npm_is_cool",
+    },
     included_files = [
         "",
         ".js",
diff --git a/internal/npm_install/npm_install.bzl b/internal/npm_install/npm_install.bzl
index 611e11cbb9..d79e259b4b 100644
--- a/internal/npm_install/npm_install.bzl
+++ b/internal/npm_install/npm_install.bzl
@@ -70,6 +70,10 @@ If symlink_node_modules is True, this attribute is ignored since
 the dependency manager will run in the package.json location.
 """,
     ),
+    "environment": attr.string_dict(
+        doc = """Environment variables to set before calling the package manager.""",
+        default = {},
+    ),
     "included_files": attr.string_list(
         doc = """List of file extensions to be included in the npm package targets.
 
@@ -263,11 +267,17 @@ cd "{root}" && "{npm}" {npm_args}
     if result.return_code:
         fail("pre_process_package_json.js failed: \nSTDOUT:\n%s\nSTDERR:\n%s" % (result.stdout, result.stderr))
 
+    env = dict(repository_ctx.attr.environment)
+    env_key = "BAZEL_NPM_INSTALL"
+    if env_key not in env.keys():
+        env[env_key] = "1"
+
     repository_ctx.report_progress("Running npm install on %s" % repository_ctx.attr.package_json)
     result = repository_ctx.execute(
         [repository_ctx.path("_npm.cmd" if is_windows_host else "_npm.sh")],
         timeout = repository_ctx.attr.timeout,
         quiet = repository_ctx.attr.quiet,
+        environment = env,
     )
 
     if result.return_code:
@@ -303,7 +313,11 @@ See npm CLI docs https://docs.npmjs.com/cli/install.html for complete list of su
             allow_single_file = True,
         ),
     }),
-    doc = "Runs npm install during workspace setup.",
+    doc = """Runs npm install during workspace setup.
+
+This rule will set the environment variable `BAZEL_NPM_INSTALL` to '1' (unless it
+set to another value in the environment attribute). Scripts may use to this to 
+check if yarn is being run by the `npm_install` repository rule.""",
     implementation = _npm_install_impl,
 )
 
@@ -390,11 +404,17 @@ cd "{root}" && "{yarn}" {yarn_args}
     if result.return_code:
         fail("pre_process_package_json.js failed: \nSTDOUT:\n%s\nSTDERR:\n%s" % (result.stdout, result.stderr))
 
+    env = dict(repository_ctx.attr.environment)
+    env_key = "BAZEL_YARN_INSTALL"
+    if env_key not in env.keys():
+        env[env_key] = "1"
+
     repository_ctx.report_progress("Running yarn install on %s" % repository_ctx.attr.package_json)
     result = repository_ctx.execute(
         [repository_ctx.path("_yarn.cmd" if is_windows_host else "_yarn.sh")],
         timeout = repository_ctx.attr.timeout,
         quiet = repository_ctx.attr.quiet,
+        environment = env,
     )
     if result.return_code:
         fail("yarn_install failed: %s (%s)" % (result.stdout, result.stderr))
@@ -435,6 +455,10 @@ to yarn so that the local cache is contained within the external repository.
             allow_single_file = True,
         ),
     }),
-    doc = "Runs yarn install during workspace setup.",
+    doc = """Runs yarn install during workspace setup.
+
+This rule will set the environment variable `BAZEL_YARN_INSTALL` to '1' (unless it
+set to another value in the environment attribute). Scripts may use to this to 
+check if yarn is being run by the `yarn_install` repository rule.""",
     implementation = _yarn_install_impl,
 )
diff --git a/internal/npm_install/test/postinstall.js b/internal/npm_install/test/postinstall.js
new file mode 100644
index 0000000000..1decbb5d30
--- /dev/null
+++ b/internal/npm_install/test/postinstall.js
@@ -0,0 +1,14 @@
+// This script is called by postinstall steps of yarn_install & npm_install rules in the root
+// WORKSPACE. It tests that the environment attribute sets environment variables as expected.
+
+const expectedYarn = 'yarn_is_great!';
+if (process.env['BAZEL_YARN_INSTALL'] === '1' && process.env['SOME_USER_ENV'] !== expectedYarn) {
+  throw `Expected SOME_USER_ENV environment variable to be set to '${
+      expectedYarn}' by yarn_install but got '${process.env['SOME_USER_ENV']}'`;
+}
+
+const expectedNpm = 'npm_is_cool';
+if (process.env['BAZEL_NPM_INSTALL'] === '1' && process.env['SOME_USER_ENV'] !== expectedNpm) {
+  throw `Expected SOME_USER_ENV environment variable to be set to '${
+      expectedNpm}' by npm_install but got '${process.env['SOME_USER_ENV']}'`;
+}
diff --git a/package.json b/package.json
index c2e1523ad4..db2c97029e 100644
--- a/package.json
+++ b/package.json
@@ -106,7 +106,8 @@
         "format": "git-clang-format",
         "format-all": "clang-format --glob='{internal/**/,examples/**/}*.{js,ts}' -i",
         "stardoc": "bazel build //docs && cp -f dist/bin/docs/*.md docs",
-        "version": "conventional-changelog -p angular -i CHANGELOG.md -s && node ./scripts/on-version.js && bazel build //:release && node ./scripts/on-release.js && git stage index.bzl docs/install.md packages/create/index.js README.md CHANGELOG.md e2e/*/WORKSPACE examples/*/WORKSPACE"
+        "version": "conventional-changelog -p angular -i CHANGELOG.md -s && node ./scripts/on-version.js && bazel build //:release && node ./scripts/on-release.js && git stage index.bzl docs/install.md packages/create/index.js README.md CHANGELOG.md e2e/*/WORKSPACE examples/*/WORKSPACE",
+        "postinstall": "node internal/npm_install/test/postinstall.js"
     },
     "husky": {
         "hooks": {
diff --git a/tools/fine_grained_deps_npm/package.json b/tools/fine_grained_deps_npm/package.json
index a9f4f0a0ef..a57c6deba8 100644
--- a/tools/fine_grained_deps_npm/package.json
+++ b/tools/fine_grained_deps_npm/package.json
@@ -12,5 +12,8 @@
     "http-server": "github:alexeagle/http-server#97205e945b69091606ed83aa0c8489e9ce65d282",
     "klaw": "1.3.1",
     "rxjs": "6.5.0"
+  },
+  "scripts": {
+    "postinstall": "node internal/npm_install/test/postinstall.js"
   }
 }
diff --git a/tools/fine_grained_deps_yarn/package.json b/tools/fine_grained_deps_yarn/package.json
index a9f4f0a0ef..a57c6deba8 100644
--- a/tools/fine_grained_deps_yarn/package.json
+++ b/tools/fine_grained_deps_yarn/package.json
@@ -12,5 +12,8 @@
     "http-server": "github:alexeagle/http-server#97205e945b69091606ed83aa0c8489e9ce65d282",
     "klaw": "1.3.1",
     "rxjs": "6.5.0"
+  },
+  "scripts": {
+    "postinstall": "node internal/npm_install/test/postinstall.js"
   }
 }