Snap caps will show a new shooter that he/she is flinching or "pushing" against the anticipated recoil. After you've done this once...you shouldn't need to do it again to "diagnose" your problem.
"Where" your shots are landing will tell the story.
Now that you know what the problem is...its time to correct it.
Flinching is not corrected (for long) by dry firing IMO.
Good instruction on how to best control recoil, proper trigger manipulation, follow through, and making sure the trigger finger is acting "independently" will ensure your technique is correct. After that, it is "mental discipline" that will keep you shooting straight.
When your shooting starts falling apart (and you know you are capable of better), go back to the basics (front sight, press, follow through) then coach YOURSELF through each shot until you're back on track.
I can't tell you the number of students I've taught that start a session poorly and lose confidence. As soon as I get behind them and am right in their ear instructing "front sight, press, follow through" the hits come right back. You MUST coach YOURSELF.
Another thing I try to get folks to do (with less success) is to shoot with both eyes open and DO NOT blink at the shot. This goes a long way toward controlling flinch. If you don't have arthritis or some other physical reason why shooting a handgun would be painful, then there really is no good reason to flinch.
It's mental, beat it!
