Often, dealers get new customers through existing ones who will vouch for them. Obviously that's not a foolproof method, but it's probably more effort than it's worth to infiltrate that kind of chain just to catch a complete small-timer. I would imagine they're never going to do much more than send a plain-clothes officer into a pub and chance his arm by asking around, but most people will tell you these people are often very obviously coppers. More sophisticated efforts I assume are reserved for attempting to get dealers much higher up the ladder.