Pick them up and look through them basically.
I'm using a pair of old Zeniths @ 12 x 50 with some light sheilds on the front and they are surprisingly good. Look for a flat field, no coma or aberration at the edges primarily. Fungus or scratched lenses, drop them. Check mechanicals, focussing, hinges etc.
What sort of price are they asking?
I got mine for $10 so no loss if they didn't work well. I actually got a second pair for nix and combined the good parts. It also gave me the optics for my guide scope.
