Hi Daniel, and welcome to the forum!
I’ll have a go at a guide to identifying fake and repro enamel signs, but I hope others will chip in (no pun intended!) with other thoughts and comments!
Firstly, there's nothing to beat coming along to a bottle or advertising collectors' fair, and handling original signs. You get to know what an original sign looks and feels like, and if you're in doubt, I'm sure one of the many collectors or dealers will be able to help you out.
Apart from that, there are some basic things to look out for. I always think the main thing to look at is the back of a sign. This will tell you a great deal. The original signs are most often grey, or sometimes dark blue, but they usually have various runs and splashes of white and other colours, and often roughly applied numbers done in enamel by brush. Beware of signs that have pure black backs, with straight rows of pin marks where they have been supported in the kiln. These are normally repros. Just to confuse you though, there are some very late but 'original' enamel signs with black backs, probably just pre- or even post WWII, which do have black backs too! (The late WH Smith's 'paper boy' signs are examples!) There are also some late but original enamel signs, such as the Players Please/HMS Invincible which have white backs!
You will soon get to recognise most of the modern and reproduction enamels produced by Garnier in the ‘80s & 90’s. Manufactured in vitreous enamel, using similar materials and processes to the original enamel signs, a large range of signs were made by them, a small number of which were quite faithful reproductions of the original signs, such as their ‘Bovril/makes contented cooks’, ‘Huntley & Palmers/Ginger Nuts’, ‘Rajah/Cigars’ and their ‘Rinso’ sign. Generally though, these can be identified by smaller sizes, black backs, fixing holes set further away from the edges of the sign than the originals, thin enamel, screen printing half-tones rather than the much more delicate lithographic printed originals. Some also have design variations from the originals, such as the price on the undersized ‘Black Cat’ sign.
There are other reproduction signs which aren’t even enamel; just print on thin metal sheet, using very crude graphics and ‘printed-on’ chipping and rusting. These should not fool anyone, except perhaps with a very dark photo and poor description on Ebay!
There have been one or two other odd repros around in recent years, such as an approximately half-sized enamelled Wills’s Woodbines packet, with a white back, and a ‘painted’ Bovril which was on Ebay recently, which even caught out one or two seasoned collectors!
Now it's over to anyone else who can add to this!
Good luck in your searches!