Use the seat of your pants!
It's there for your judgment.
If the turbulence is bad enough that you tighten your seat belt you should probably slow to the TAP speed.
Or, maybe you inadvertently penetrated a thunderstorm. Good idea to slow down--or speed up, as the case may be!
Assuming you're American, I'd suggest reviewing the AIM for the classifications of turbulence. Anytime you experience moderate or greater is a good time to fly at TAP speed.
Good luck.