How do I tell the good cars at an auto Auction?

You cannot. Even if you do an OBD2 code check, you don't know if they erased codes just before pushing the vehicles into view. I saw so much pure junk at auction I never went back. Nearly all of them had oil in the radiators. Picking a car at an auction is like looking for a wife among recently arrested steetwalkers

There are no good cars there. If there is a good car it's bundled with several other POS vehicles so you lose every time.

Most cars at an auction are pretty bad.  You should be prepared to spend a lot of time/money on one.

You can only do a visual inspection.  So you take your chances.