Nicks, dents, chips, bends.
On the underside, you want to look at the three locking nubs that hold the props to the motors. look for stress cracks, just something that doesn't look like a new prop. I always check the amount of "lay", hold the motor bell, and check the "play" on the prop., you can have a very small amount of movement, but again, if it feels more than a new prop, change it.