This Minecraft datapack allows you to visually inspect a crop to see if it is fully grown and ready for harvesting. Simply hold any hoe in your main hand and look at the crop.
Only air blocks are allowed between you and the crop, so looking "through" other crops or standing on top of the crop may be difficult. Step up one block if it's hard to get a clear line of path between you and the growing crop.
There is also a version of this data pack that gives you a message when the crop IS ready to be harvested.