I'm not an expert on collecting signs by any means but do like vintage advertising and antiques. There is a learning curve like anything. Once you do it long enough you can generally spot the reproductions a mile away. That is probably the easiest part. Reproductions are often too clean, made of thinner metal and show up a lot more often. Often sized differently than the originals also. I find it easier to just avoid even original examples if they have been reproduced. Mainly because the reproductions kind of cheapen the experience of owning an original as well. Even if it's a cool sign I don't want one if I see an example of it in every garage/gameroom.
Here is a pretty cool looking sign (see pic). It's a reproduction though. It's too clean for one thing but also you see a ton of them for sale in all different sizes from Knapik's Marine. I'm guessing there may even be some originals somewhere from Knapik's Marine but who'd want it when all the repros have the same text.
Fakes are generally reproductions that have false or accelerated patina. Printed patina can be easily spotted as it looks rusty but feels smooth. Some signs are made new to have an aged look. Other times guys will bury signs in the ground to get some rust going. Once again it helps if you have a pretty good knowledge of what signs have been reproduced. A lot of the reproduction signs have those brass eyelets in the corners for hanging. So that is something else to look for.
As far as junk goes. Like any hobby there are always starter pieces. Cheaper pieces that have less value because of condition or being common. If you buy smart it's hard to really get burned on these because there is always another guy starting out that will by your cheap signs once you move up to the better stuff.