Not sure what you mean. Pressings, using the word properly, are just subsequent prints in the same print run (an example are the two different versions of Iron Man #15 or the FF #52 with the ink glitch in a plate, while a portion of the run does not have it). You must be referring to early reprint like the Golden Record, if so that is easy to spot as there are differences on the cover.
I was talking about reprints for sure. I'm considering a big buy of Fantastic Four 1 and want to be educated before I pick one up. I want to understand how to buy one that hasn't been retouched etc....