As said, skip the spray paint.
If you want some sort of visual on the edge and if you have the 'correct' sharpening angle, the Sharpie smear on the edge is quick and easy and works fine. Cheaper and easier than spray paint too.
The level of 'sharpness' to get or go for depends mostly on the sharpening angle. A lower angle is 'sharper' (but more delicate), while a steeper angle is not as 'sharp' (but generally more durable). example: straight razor (very low sharpening angle) versus an axe (steeper angle).
The 'correct' angle to use depends on the use of the tool (razor versus axe, for instance) and also on the specific steel alloy and its heat treatment ('better' steel can often be sharpened to a shallower angle and thus be 'sharper' while still keeping an acceptable level of durability.
As to testing the 'sharpness', there are multiple ways to do so. With the edge marked with the Sharpie, look at the edge and see if all the ink was been sharpened away and the edge angle on both sides of the blade meet at a single line. Bright even light and a magnifier can help there. Also see if there is a folded-over burr from one side of the blade flopped over to the other side. A burr can be wicked sharp, but once it breaks or folds flat or back over, the 'sharpness' can be gone.
Then you can
carefully see if the edge 'glides' or cuts/sticks when attempting a very-very-very gentle "push cut" against a thumbnail (or a small wooden/plastic dowel or similar) at a 'reasonable' cutting angle (do
NOT do a "draw cut" against body parts, you can get cut!)
Once you start to get a 'feel' for sharp knives and are
careful, you can sort of do the gentle almost-a-push-cut (barely push against the edge) against a finger tip/pad and see if the knive edge is 'grabby' or 'sticky' against the skin (if so, it is getting or is pretty sharp and is trying to cut into the skin) or if the edge more like 'slips' or slides against the skin.
NOTE: Be careful!!!! If you are a bit ham-handed or don't have experience with SHARP edges, don't test an against against body parts. A -sharp- edge (or a sharp burr) can cut with with very little force, especially if you do a "draw cut".
Or you can do the above mentioned cut through a piece of paper. Hold the paper in the air and using the edge of the knife, slice through the paper and feel how the edge does. Did it smoothly and evenly slide/glide through the paper as it made the cut or were there spots or sections of the blade where the cut felt different or didn't cut smoothly (or at all).
Shaving the hair off your arms can work too, but after a while you then don't have any hair left on your arms.
Also see above about draw-cuts against body parts.
