What should I look for when buying a bike from an auction? I'm going to an auction to look for cheap gems. Each bike is shown only very briefly and the bidding starts immediately. No hands-on inspection is allowed.

What should I pay attention to when trying to decide how much / if to bid? Are there some immediate signs that I should look for that either hint to the high or low quality of a bicycle? 

I don't know brands very well and the brands here in Finland might be different from other countries. However, brands I should look for or avoid are welcome, but any other more general external indicators for quality would be better.

I know this is similar to What should I look for when buying a used bicycle? but I think the situation is different enough to merit a separate question.
Just like with any other area of expertise, there is no substitute for years of experience. If you need a simple heuristic though, brand recognition in combination with a general knowledge of groupsets and their quality can go a long way.

At the very least, know how to tell forged components from stamped. Learn Shimano's and Suntour's groupset lineups. Groupsets are generally a good indication for the overall quality of the bike. 

Know about the different qualities of steel (hi10 &lt; 4130 chromoly &lt; branded {reynolds, columbus}). Learn about the qualities of good steel lugs (some framesets are worth money that won't have any parts on them).

Back all this up with a smartphone and you should be good.