Probably not much of an answer as i don't know the details of the process behind it but yes, you can definately test for gear use through the testing of hair.....
Googling will probably tell you more but it is a standard test now in professional cycling events....the UCI website may have some info.
Or if you happen to have Lance Armstrong's number i'm sure he can fill you in as i understand he had his hair tested on 6 ocassions just prior to an during the 2010 Tour De France.
With hair follicle depending the AAS I'm sure its possible. Someone would have to be doing some in depth testing or to prove a point to do hair follicle.
i had to give a hair sample 5 years ago and urine for my current job.at the time i was on test and deca.there was a couple other guys there that were on juice that just walked out of the interview when they heard had to give hair sample.i think they are looking for mind altering substances vs juice