What to look for when buying a frame?

[T-Type]

In light of the shoddy workmanship thread and the great tips offered up by fesler and others I was thinking about those of us looking for frames. I'm sure most of us have been there either starting off with just a frame or finding an alternate for a project currently in the works....

either a rolling frame or one that is just bare, powder coated/painted or not.....other than the obvious signs of wrinkled metal or welding seams of added on metal...

what do you all look for? how can you tell it's rolling straight and not tweaked? typical rust spots? insight appreciated.