For me it really depends. Most any way can be good. But some that really stick out to me include:
Finger in through the leg hole to test for wetness, but only if I'm wearing a onesie or shorts and not in public. Alternatively, touching the outside of the diaper in the same spot should get the same results.
A gentle, discrete bum pat if we're in public and there's a smell, along with occasional whispers in my ear asking if I'm wet.
*blush* A visual inspection of the front, in private only, to see if I made it to the potty on time.