I have come to believe that i know alot about Ds (undercover cops), some of the things i say here generalised. My experience is with the ones that work raves in NSW and they do not look like cops at all, alot the males are non-white, alot of the girls though seem to be white. One of the ways you can pick em is that they do often go around in inter-racial couples. Another thing is that they dress 'credibly' very clubber like, they don't stand-out because of the way they dress but i've never seen someone i've thought was a D dressed up in kandi or "out landish" raver dress
The biggest give-away is that they do not use drugs this is something i'm sure of, they can have good excuses for this however which they may 'flag' to the party, i've never seen one pretending to look smashed. As far as behaviour i've seen them chilling in a quite spot, chilling in a spot with a very good vantage point for everything going on around them aswell as somewhat obviously (though again 'giving off signs to the party' that they have reason for this) standing around in the middle casting an eye over whats happening around them. There reactions may give them away, i've seen one go very wide-eyed and fix her gaze on me when i was doing something innocent that looked like i was dealing.
The ones i've come into contact with are far from stupid in the way they act and they way they talk, very natural and friendly, in the story i'll tell below i was very convinced for a fair while they weren't a D. From the convo i had i can tell you they know their facts about drugs such as street-names, other slang, prices, effects etc but if you talk to them you can pick on little hints that they know the facts like they've learnt from them book not picked them up from lots of conversations, phrasing things just a little differently or saying things that normally go unsaid, they also know a bit bout clubbing/raving though the one i talked too made a slip-up revealing they couldn't have done what they claimed (this only slightly raised my suspicion they were a D at the time, i thought they were just confused).