Back in the day, different consoles and games were made for different parts of the world. PAL was designed for Europe and Africa, NTSC for the Americas, and even a few others were made. Those games and systems ONLY work in those parts of the world, so how can you tell which region your product is from? 

For Nintendo, they put in a handy "USA" or "Nintendo Of America" on their products. PlayStation and Xbox both have NTSC (America) or PAL (Europe). On PS3, they have a "US" suffix on the spine of their game cases!
Let's take a look at some examples.

Nintendo

NES Catridges - (NES-UM-USA-1)

SNES Catridges - (SNS - MK - USA)

N64 Cartridges - (USA)

GameCube Disc - (Nintendo Of America/ -USA)

Wii Disc - (Nintendo Of America/ -USA)

Wii U Disc - (Nintendo Of America/ -USA)

PlayStation

PlayStation (PS1, PSX) - (NTSC)

PlayStation 2 (PS2) - (NTSC)

PlayStation 3 (PS3) - (BXUS)

Xbox

Original Xbox (OGXbox) - (NTSC)

Xbox 360 (X360, 360) - (NTSC)