If you're a causal player or casual buyer of retro stuff, particular the Game Boy, it's easy to end up with a fake, or reproduction cart. This is a really short guide on what to look for if you want to be sure if it's original or not.
This is a non-exhaustive guide, intended for the uninitiated :)
Before we start
It's worth saying that in a lot of cases, if you just want to play the game, a reproduction cart might be what you're looking for. For instance, an original copy of Amazing Tater or Shantae (both great fun) might be a little beyond what's considered affordable.
In many cases, a cheap reproduction cart from somewhere like AliX are nearly always flashable, which means regardless of what Game Boy ROM has (possibly illegally… depending on the country) been put on the cart, you can flash your own game and use the cartridge for your own distribution.
The fastest way is usually the screw that holds the shell together. An official cart will have the screw at the bottom of the shell and it needs a "gamebit 3.8mm" screw driver to open it (or apparently a melted Bic Biro).
Below you can see on the right is the original screw location and type, on the left a common reproduction cart.
The photo above also has the "Made in Japan" missing on the left reproduction cart.
The front can be trickier, especially when stickers are well made, but if you can see it, you want to look for an embossed number on the sticker - this will indicate that it's original. However, you can find that the emboss is missing on an original and in some cases you might have an aftermarket sticker to replace a badly damaged sticker.
If you're able to get different angles of the cartridge, then the follow might also be useful.
It's not uncommon for a reproduction cart to use a very different (and cheaper to produce) PCB, and so in quite a few cases the bottom of the cart will be cut out to make room for the chips on a reproduction cart:
Finally the indentation mark and number found on the inside front shell, from what I've seen, is always a sign of an original and legitimate cart. I first heard of this on Reddit (but can't find the original link) and if you can see that angle, it's pretty faultless - plus it works for Game Boy Color and Game Boy Advance carts:
So that's it. Hopefully useful to the tiny venn overlap my web peeps who usually follow my blog and those who might also be as interested in Game Boys as I have been in the last year!