Welcome! That's a big can of worms. There's a lot to look out for. If you're collecting a specific brand, get to know the brand and logo changes over the years of that brand. There are a lot of fakes and reproductions out there. If the colors don't look right or the logo looks a little off, it's not original. Don't go by the patina either. There are people out there taking new reproduction signs and cracking the porcelain corners around rivets and dinging them with a hammer...then sticking them outside to rust, and/or spraying with salty liquid to make them rust faster and look old. It's big business.
In my experience, double-sided porcelain signs are harder to replicate, so those are a little safer.
If you find signs with cute ladies on them, the signs are new. Old gas signs come from a very conservative era when it was typically just the logo and/or a mascot.
Get to know the shape of the signs you're collecting. Fakes/reproductions sometimes get the shapes wrong. Sometimes they have the incorrect number of mounting holes. The mounting hole grommets are a tell also, old signs typically had steel grommets...not aluminum. Look on eBay and look to see if the sign you're looking at is selling in a lot of auctions. It there are a lot for sale, it's probably not original, since there's a lot available. Also do an "advanced" eBay search for "sold listings" for previous listings. Most reproduction companies are legit and use the term "vintage style", that's a great indicator that it's not original.
If you're just collecting, collect what you want, not necessarily what you think you can resell later. But make your decision on what condition signs you collect. If you only collect mint condition signs, they will fetch more money if you ever sell them. On a scale of 1-10, I collect Union Oil conditions 7to10. I collect Union Oil cans in condition 3 to 10.
This is all from my experience with collecting Union Oil memorabilia.