Best thing to do is talk about numerous things with people that only people over 25 would know about. It's not to hard to figure out who the questionably young people are, but alas, that means you have to get to know people before getting naked!
Excellent advice! Here are some tips to get you started.
Start every sentence with "In my day...", or in every room say "The music today isn't as good as it used to be - it is just a loud noise" unless it is classic rock or classical... Failing that, and this is sure fire way of age verification, try this one...
"Remember when we had dial up internet? My parents used to get mad when they wanted to make a phone call... 'Would you get off the damn internet!!', You know, those really old folk?"
If you are met with a blank stare (normally a silence because a stare is not a thing in 3DX). They are probably underage or they think you are boring. Either way, it is best to avoid.
Source: Saya is borderline old and proud child of the 80s! Dial up internet, floppy disks and Sega Megadrives FTW!
Disclaimer: Saya cannot be held responsible if this advice does not work and you end up in jail.