Low karma profiles with no post history are usually the tipoff people look for. But you'd be surprised how often someone tries to phish your account or straight up purchase it with a middling amount of karma (20-100k) and I'm fairly certain a lot of those are for the purposes of discrete marketing.

Sometimes you can find someone doing a piss-poor job of shilling something, look at their post history, and see the exact moment they sold or otherwise lost their account. Just a sudden shift into singular topic shill posts about a topic never found in their interests prior.