-Grab a sheet of white paper
-Rub the piece of paper under the plant
-Brush your hand over the area on the paper which you rubbed under the plant
If you see a reddish smear across the paper, then your plant has spider mites.
I'm not too sure how effective this could be with Flytraps but I've used this method for a long time, maybe it will help someone catch the problem before it gets any worse.
Jiu Jitsu is a way of life.