Check the eyes and stomach: if either (the eyes particularly) are sunken in, then pass on the fish. Check to see how they're maintaining the fish, primarily the presence - or lack there of - of driftwood. If there is wood in the tank, pick a fish that's chewing on the wood; if there isn't, ask them to put a piece in and watch the tank for a bit. You want a fish that's actively working on the wood.
As far as diet, the Panaques are primarily wood eaters. It's a must. Aside from that, just offer a well-rounded diet, and see what it likes. Start with zucchini and/or yellow squash, then go from there.