Okay, here's the key:

1. Find out if they are engaging in human sacrifice.

2. Find out if they swear blood oaths.

3. Find out if they employ the uses of rodents in what could be termed an unusual manner.

4. Find out if they hate football and are bored by hockey.

5. Find out if they bow down before graven images of William Shatner.

If so...prepare to mount an extremely hazardous rescue operation. Your friend's sanity could hang in the balance.

Several other disturbing signs:

a)A reduction of interest in watching TV.
b)A reduced tendency to play computer games till 3 AM.
c)Going to bed early and rising early.
d)Practicing Qui Gong.
e)Meditating.
f)An alarming tendency to read "serious" books, as opposed to crime novels.
g)Totally not giving a shit what anyone else thinks about all of the above (a-f).

These disturbing sings could indicate that your friend is slipping rapidly out of the grasp of what is termed "normality". It may be possible to save him/her, but it will not be easy. Be prepared for the worst!

***(Lest anyone think I am being insensitive in bringing humour to this matter that Raptor has brought up...well, I know the friend in question...and that's why I am doing so. I have reason to.)