A general rule of thumb is to look on emovieposter or heritage database. If the poster regularly sells for under $50, it wasn't worth anyone's time to reproduce it.
If the poster is pre-90s, and is 27x40 it is a repro. If it is 27x41 it is likely legit, unless it is one of the heavily pirated ones.
Look on ebay for the poster and see if anyone is selling full sized items marked as reproductions. If they aren't out there, the odds that this one a is repro or forgery go way down.
Look on Mel's authentication site or the lamp one for known forgeries.
If it isn't a popular or cult title, it likely isn't a forgery
Anyone else want to chime in?