With exposed 120 film, the backing paper is your best clue. Many rolls have identifying text, codes, or distinctive backing-paper colors that can help you tell films apart. If needed, in complete darkness or a proper changing bag/darkroom, the roll could be wound back toward the start to look for printed identification on the backing paper. You can also search any numbers or letters printed on the paper.

That said, there may not be enough information to determine the exact film and speed with certainty. A lab may be able to infer whether a roll is color or black-and-white, so it’s worth telling them you have a mixed batch and aren’t sure which is which. In practice, you may still have to make an educated guess for some rolls.