If you are shopping online, the BB size usually shown on the specifications section. For example:
24mm BSA for 2 pc. design, 73mm
If you are shopping locally, just go ahead and measure it to confirm.
The majority of mountain bikes sold today have either 68 or 73. The higher end bikes, especially carbon fiber, may have press-in BBs with odd widths. Those won't work. Look for anything with an external cup BB and it should work just fine.
Be advised that the BBS02 works out of the box for 68mm. 73mm will require some spacers and longer bolts that you can get at just about hardware store.
Fat bikes are almost always 100 mm or greater BB, so avoid those.