I figured out how to tell if a friend is AB/DL.
If diapers have more meaning to an AB/DL and also if your friend is secretly an AB/DL, then they will probably act weird if you mention it. (Reference: http://www.adisc.org/forum/diaper-ta...le-saying.html)
You can also look for any small hints. For example, I have a friend that acts a lot younger than he should at times and sometimes even does "baby talking" to be funny.
The next time you see a friend that you suspect to like diapers, mention diapers and listen carefully for a crinkling noise.