Luckily there are many common elements that make them relatively easy to spot and avoid. Which i will cover in this guide.
Pokemon games: First thing to know is the game titles themselves and how the cartridge looks
Pokemon Blue (Blue colour cartridge)
Pokemon Red (Red colour cartridge)
Pokemon Yellow (Yellow colour cartridge)
Pokemon Silver (Silver colour cartridge)
Pokemon Gold (Gold colour cartridge)
Pokemon Crystal (Clear blue colour cartridge)
Pokemon Ruby (Clear red colour cartridge)
Pokemon Emerald (Clear green colour cartridge)
Pokemon FireRed (Opaque red colour cartridge)
Pokemon LeafGreen (Opaque green colour cartridge)
Pokemon Sapphire (Clear blue colour cartridge)
All the above colours are the only colour these games came in so for example if you see a Pokemon Silver in a black cartridge or Pokemon Ruby in a black cartridge... its fake.
Also you will see "made up" games like Pokemon Jade, Chaos Black etc .... these are all fake and are made up from bits of other Pokemon games and if you play one you will see the badly translated English in the dialogue which often makes no sense!
Every genuine Gameboy game has a code on the label for example Pokemon Fire Red is AGB-BRBP-EUR. The first part is the system the game is for AGB = Advance Game Boy other codes are DMG = Dot Matrix Gameboy (original) and CGB = Color Game Boy. The second part of the code is the game code. The third part is the country code, EUR = Europe, others include UKV = United Kingdom, USA = USA and JPN = Japan etc.
99% of fake games i have seen have the country code as USA mainly because they were intended for the American market.
This doesn't mean that all USA games are fake, just means you need to look closer to see if it is genuine.
A sure fire way to see if an Advance game is genuine or not is to look at the bottom part of the circuit board that is visible on the cartridge. ALL GENUINE ADVANCE GAMES WILL HAVE NINTENDO STAMPED ON THE CIRCUIT BOARD. This is the Nintendo logo and some code/ serial numbers. If it is not there it is fake!
Also look for the obvious, game labels often look wrong, as in the picture may be stretched, blurred or badly stuck on. Compare the picture to a known original, sometimes the Nintendo seal of quality is not there or in the wrong place etc.
Another give away is "new" games being offered as cartridge only or boxed without instructions ... if it was new and genuine it would have both box and instructions??
If there is box and instructions (which some fakes do have) always look at the instruction book, genuine instruction books (or trainers guides) are just a little smaller than the box, so will fit perfectly in the box. Fake instructions are half the size of the box, so if buying from a picture its an easy way to spot a fake. Closer inspection you will see the box is inferior quality and often badly printed.
The original Gameboy games and Color games have the same traits, except the printed circuit board part is only visible if you open up the cartridge. But the USA prefix, the size of the instructions etc all remain good ways to spot the fakes for these systems. Also the cartridge screw on the back will be a security screw and not a normal philips head screw or triwing screw (triwing screws are only used on the Advance cartridges)
Fianlly if its claiming to be a "new" game, just look at the price...genuine Pokemon games that really are new and sealed go for a high price (£50 upwards), so if you see a Pokemon game that claims to be new, comes as cartridge only or has no instructions and is cheap then you know to steer well clear, after all you get what you pay for.
There are many good sellers here selling quality genuine games, but also a fair few of the not so good! We only sell genuine games and any pirate copies we come across are destroyed. If no one bought the fakes then they would stop making them. So make sure you make the right choices, happy hunting :)