1. If it sounds too good to be true, it probably is.
2. See if there is a warranty
2.a If there is a warranty, see if it is honored by the manufacturer
I've been to a couple of them, and there really aren't any decent deals on hardware that I couldn't find from a reputable dealer that would back their product. That said there are some really decent deals there on software, books, and oddball stuff.
For example - I walked out of the last computer show with about $250 in books for under $70. Not one of them was more than a year and a half old. I also nabbed a couple powered antennas for my TV's for $5.00 a piece. The same exact ones at best buy go for $30. I more than paid for my admission on the savings with those.
The age old phrase stands true: Buyer Beware.