It bears pointing out that there are different types of shills, too.
The noob shill is easy to detect. He has a generic, topical user name like "drugbuyguy", or maybe something nonsensical. Sometimes the user name is in ALL CAPS. He has been a member of the board for 3 days, and he has 8 posts, in different fora but all with the purpose of unreserved endorsement of the same as-yet-untested source. Sometimes the endorsements are even OT in another source's thread. This type of shill is usually caught and banned immediately, perhaps roping in a few unsuspecting noobs before he gets a chance to go on another IP and create a new account.
The pro shill is a lot more difficult to spot. She will be a long-standing, relatively trusted member of the board. A shady vendor will contact her, offering a very sweet deal if she agrees to use her credibility and social standing in the community to endorse him. Maybe he will send her a few samples, too, and they'll be good. "What's the harm?" she might think to herself? He clearly sells real products, and they're good, and no one will need to know that she's getting a little on the side for each order. A few more solid early orders later, the whole board jumps in, and the "vendor" takes everyone's money and runs. Sometimes the pro shill will get caught, and a drama bomb will go off, but sometimes not.