I think all of us with an interest in vintage graphics have been fooled by the odd ‘too good to be true’ purchase that turned out to be a repro

I would put forward a simple 2 step process

  1. size : is the poster a standard ‘old money’ size, many many repros are either smaller than the original or conform to a ‘modern A1 etc sizing. Its a fact even today that non standard sizes are very unpopular with printers.
  2. paper: is the paper rough and thin enough to support the stone litho process. One key way is to view the poster against a back light from the reverse. There should be an obvious view of the front in mirror. Most repros are on glossy thicker paper

There is also the well known method of viewing the offset litho dots trough a glass. This can be tricky over the net though and also not all post war posters conform.

So keep it real and make those checks before making an expensive mistake.