How do you tell the region (America, Europe, Japan) of a Nintendo 3DS game from its case? A Wii game usually has "PAL" on front side if it's an European release, but how does a 3DS game indicate its region?
You can actually know not just the region, but often the individual country the game was released in.
Look near the barcode in the lower right on the back of the case. You'll see a code like
This is the same code that's on the cartridge itself, though that one will be prefaced with
I'm not aware of any sort of unambiguous region marker (though there could possibly be something in the fine print on the back that I've never noticed). At the very least, though, the ratings board designation could be an indicator, as to the best of my knowledge, almost all U.S. games will carry an ESRB rating; likewise, Japanese games will carry a CERO rating, and European games will carry a PEGI rating.
While it's technically not a guarantee, I know that most retailers in the US will only sell games that have ESRB ratings. If the same is true for other regions, it should be a relatively reliable thing to be able to check for.