Things like dings should be fairly easy to spot. But you need to check to see if they have been repaired well. Check for water penetration or if the repair is watertight.
Check the fins. If they've been repaired are they still in line and at the right angles and are they still well fixed in? If the board has a fins system make sure you have some fins to try and that the retaining screw threads are still good. FCS threads can where out and if they do a new plug has to be fitted as you have no way of securing the fin.
Check the deck for pressure dings. Alot of boards get damaged on the deck due to popping and this can be concealed by wax. Take a credit card with and don't be afraid to scrape off some of the wax to have a look.
Also you probably want to avoid a board that's about to delaminate or has alot of water penetration under the glass. Look for really brown areas of foam around ding repairs. This could be an indication of water penetration.
A board that is delaminating will probably have a yellow tinge to it all over and if you lightly squeeze the glass it will give and make a noise just like foam pulling away from glass, a light tearing noise because that is exactly what is happening under the glass.
However on the plus side if you can glass yourself then a delaminating board with not too many dings can be a real bargain if it's a nice shape. You can buy them for next to nothing and then de skinning them is so easy as the glass just falls off without damaging the foam too much. Then you can re glass it and you have a newly glassed retro board.
KS