Biggest thing I check first is the inside of the bowl to see if there are any cracks and how the draft hole is drilled. I also like to smell the bowl and see what ghosts may have taken up residence in there. Then I check the stem for chatter and how well it fits.

Brand and overall condition come into factor at some point as well. I mean I'll pay a lot more for a beat up Dunny or Sasieni than I would a Yello-Bole or Dr. Grabow. Use good judgement.