Do the usual checks. Look under the TRC. Check for cracking and/or delamination of the fingerboard around the nut and under the first fret. Check the neck for warps and twists. Take a peek under the guard if possible and look for any conspicuous messing about.
"Which things should I check before I decide to buy it?"
What is the seller/dealer's return policy, that's the first thing I'd check! Then all the usual things. I'd be surprised if he will open the bass up at the show, but hopefully he will.
