I would think that any child wearing underpants - rather than diapers - is old enough to say for themselves if they are wet or not. I never did that to either of my boys when they were little. I would ask them - and at first it was obvious that they wet because they soaked through. I don't ever recall inquiring about whether they were wet or not in front of their playmates - I think that would be embarassing.

For children wearing diapers who are almost ready for underpants but just not there yet - it is not easy to determine if they are wet or not because the diapers usually absorb.