As stated above, Imbel receivers are marked "Imbel" somewhere on the side of the receiver. Some have a logo that looks like a Gear, some do not, but they are all Imbel that state so. Most will have an Importer stamping as well, to tell who imported it. This is not really important, just the "Imbel" stamping.
I don't know of any FAL receiver that is not marked by the manufacture. It is required by law that they be marked, so if you see one that is not, I would leave it lay
I acutally have a beat up old thumbhole Hesse FAL, with a pitted barrel (it was full of gunk when I bought it and it was my first FAL). I put some US parts on it, restored it to original configuration and it runs fine, and I can hit steel gongs out to 500 yards with it, 15 out of 20 shots, so it does OK. The Hesse receivers are just suspect, less value and are not worth trying to fix, if not operating properly. Barrel timing was a big issue with them, as I understand it. I really doubt you will find a Hesse anymore, but they could be floating around.