Do you have a shop that you use regularly? I was in the same position and I called my favourite shop and asked if I could put the wheels on his balancer for inspection. I made sure he was aware that I would be asking him to mount my tires if I buy the wheels. So I set up a meeting with the seller at the shop. Good thing too. 2 wheels were bent.
You could also bolt them to your car one at a time and give it a spin. Make sure you check both inside and outside. Also, use a tool as a reference point. I like to place a screwdriver laying on it's side on top of a block.. maybe a 2x4
Point the screwdriver tip at the edge of the rim about 2mm away and spin the wheel. If the wheel has any defects, it will be easy to see them. Without the reference tool, it's very difficult to detect bends.
Have fun and good luck.
E39 528i Touring - Unicorn Package (5 Spd M)
E39 M5 - for Sunny Days