Research, research, research.
Research 1: The watch in question. Does the case and movement match? Is it a particularly popular vintage model that is fairly easy to fake (i.e., Rolex, with its ubiquitous design and difficult-to-remove caseback). Is it a watch company that is now thought to be high-end but at the time of the vintage watches wasn't (these motivate the unscrupulous to buy several cheap old watches, shine them up, and mix and match parts to make a watch they can sell to those wanting a high-end brand at vintage prices.) Is the quality up to the standard of the company? Is the movement correct for the markings it displays? Did that watch come with that movement? Are the case markings consistent with the movement and watch? I once saw a fake/marriage Ebel with a technical reference that indicated a different movement than what was in the watch. I have many other times seen used Ebel chronographs marketed as having Zenith movements (Ebel's caliber 134) when the case showed a caliber 137 (Ebel's version of a Lemania movement). It is wise to do this research as part of the hobby, before even looking for watches.
Research 2: The deal. Is the price reasonable for the watch? Is the provenance credible? "Too good to be true" usually is, unless you can study the watch itself with some expertise, or validate it with an inspection done at an AD. But I've never taken a watch to an AD for an inspection, and still bought many vintage watches with confidence.
Research 3: The seller. Is the seller reputable? This is not always easy to ascertain, but doubt caused by lack of data in this category is magnified by doubt in the two categories above, and slightly ameliorated by confidence in the two categories above.
I would avoid brands about which you have not done that research yourself. Many have posted photos here and asked for advice, but frankly that advice is often wrong. I've seen many watches that looked fine that were accused of having misaligned dial markers or dial paint on the edge of applied markers when it was really just the way the watch was lit, the perspective, or even distortions through a plastic crystal. I've also seen those who expect the perfection commensurate with the modern reputation of a company from a time when that perfection was not as easily attained. But I've also seen positive responses for watches with obvious mis-matches of components that I knew about because of my narrow expertise with that particular brand. Getting advice online provides only a little risk mitigation.
Rick "research, research, research" Denney