Try to slice a piece of paper.
A good sharp knife makes a clean cut.
A dull knife makes either a ragged cut, or worse, just pushes the paper to the side.
I like this article: https://www.fieldandstream.com/articles/hunting/2014/08/paper-cut-testing-blade-sharpness#page-4 and YouTube
is full of videos of people showing off their knives via this test.
outdoors.stackexchange.com/users/9109/cobaltduck
The back of a fingernail can be a good rough indicator while you're sharpening - see if it "catches" when you apply very gentle pressure at ~45 degree angle.
This can also be a good way to check if the edge still has any dull spots.
outdoors.stackexchange.com/users/16788/tempest_col
The best method in my opinion is one that must be learned, and involves moving your thumb across the blade, perpendicular to the blade not down it !
It's very difficult to describe the feeling
but if you do it on enough dull blades and sharp blades you will begin to be able to tell the difference.
One other thing I will do if im sharpening a knife for a friend or coworker to display how sharp it is is shave a couple hairs off the back of my hand.
Lick your thumb, moisten the patch on the back of your hand and shave as you would with a straight razor.
(You may want to wipe the knife clean after this.)
Sounds weird
but i do it because your average joe will know its extrememly sharp if you can shave with it and be careful with it.
One other test could be to attempt to very thinly slice a tomato, with and exceptionally sharp knife you should be able to get nearly paper thin slices with a light sawing motion.
outdoors.stackexchange.com/users/9054/Nate W
Slice a tomato with very little pressure.
This is almost always when I notice a knife needs sharpening, and the difference is profound.
Edit: I should have added ripe tomato; a sharp knife will slice it, whereas a dull one will smash it or rumple and tear its skin.
outdoors.stackexchange.com/users/17581/Oscar
Generally I categorize sharpness into 3 levels: The tomato test - being able to slice through a tomato skin.
Not use the pointy end to break the skin and then slice but directly slice the skin all the way through.
This is sharp enough for cooking.
The paper test - being able to slice thin phonebook paper or magazine paper or newsprint.
For general carving knives I find some tearing after the knife has started a cut to be acceptable.
If I really want sharp then it should be able to cut paper with zero tearing.
The shaving test - you should be able to shave with it.
This is literally razor sharp (otherwise known as "scary sharp").
I've only been able to do this on some knives and I wouldn't want all my knives to be razors anyway.
outdoors.stackexchange.com/users/8914/slebetman
Cheap, disposable sponges.
Unlike paper, they can be tested on while wet, which is a big advantage when sharpening with waterstones.
Don't use the scrubby side (some people like using it as a deburring aid, though) Something dull will not be able to cut into a sponge.
Something sharp will easily slice into it.
Something d...
sharp can push-cut into the corner of a sponge.
Something b...
y
damn sharp can be laid on a sponge and pushed in (a razorblade easily will!).
outdoors.stackexchange.com/users/8657/rackandboneman
If you're sharping the knife outdoors (where a tomato is not available), a good test is to wield it perpendicularly to a grass stalk and see if the knife cuts it off.
outdoors.stackexchange.com/users/7982/Dmitry Grigoryev
You could just cut of some hair.
No need to cut other resources and its safe because yeah its hair.
And for any type of baldness you can adapt the amount of hair you would like to test on.
outdoors.stackexchange.com/users/17612/Snickbrack
In order for knives to be useful, they need to be sharp. If I am sharpening one myself, how do I know when I am done?  What would be a simple easy test of whether a knife is sharp enough?
How to test the sharpness of a knife?
outdoors.stackexchange.com
<knives><knife-sharpening>
outdoors.stackexchange.com/questions/21828
outdoors.stackexchange.com/users/8794/Charlie Brumbaugh
The tomato test is very common. Other tests include trying to cut grass or a cheap kitchen sponge, shaving, cutting hair, scraping the back of a fingernail or slicing a piece of paper. Try to slice a piece of paper, shave, cut some hair. You can use your fingernail or thumb. Slicing a tomato, using sponges and grass also work.
Sponges and grass can be used. Slicing a tomato is a good test. Shave, cut some hair or use your fingernail or thumb. Try to slice a piece of paper.