Some are really easy to spot, some are a little harder and some are very difficult to see. The obvious things to look for on the outside: A lot of fakes just say "Game" on the concave part on the top, several fakes don't use the security screw but a Phillips head instead, some games aren't quite the right dimensions and as such have a very tight fit when inserting into the Game Boy, and of course label printing issues such as cropping, no Nintendo Quality logo, an ESRB rating (if the game had one originally) being very obviously faked.
Of course, the easiest way to tell is to open the game up and look at the circuit board. You want to see the ROM with the same game code as the front label, just like NES and SNES games. Here are some pictures of the examples I mentioned:
https://www.dropbox.com/s/yklnmwm...
https://www.dropbox.com/s/almpb9u...
https://www.dropbox.com/s/epcf1y6...
https://www.dropbox.com/s/3mgc25q...