I just assume everyone has pests and treat frags and new pieces accordingly, but as Emac mentioned, observation is key. Know what to look for and if you can, look at their tank. Fish should look healthy, just like the corals, there are more than just coral pests to deal with here.
I stay away from people who try to sell more than what you're looking for (although, some have a lot of stuff to get rid of). Generally, a good reefer will tell you to look around and see if anything catches your eye, they're not trying to sell you everything, but kinda opening the doors and being neighborly. I'm old school, so if I'm actually selling something, chances are you'll walk away with more than what you came for and it won't cost you a thing. A red flag to me is someone who doesn't bother to ask you about your tank/system or cares where the frag(s) are going. Not me, I usually ask you more questions than you'll ask me. I took the time and effort to care for and grow these animals and I want them to go to a good home.
Also, trust your instincts, people give off vibes, so trust your gut. Get to know other reefers, join a club or use forums like this and you'll see there are some who are here to help and have a good chat about the hobby.
Do some research or buy some used books off of Amazon about corals and the hobby. These books will be loaded with pics of healthy corals, as they should look without the blue light special getting in the way. They will also be full of useful information that often gets overlooked in the I'm-an-sps-guru-but-only-been-in-the-hobby-for-5-years-insta-reefer thing that's currently going on.
Be patient and take your time. Know what you want to buy and have a good reference for what it should look like. Ask for colony pics. If they don't have any, good chance it's not something that's been in their tank for long or they're trying to move pieces. There are a lot of homeboy-wholesalers out there, so be aware.