If you are Keeping the drive train the same, not changing the crank, then a visual comparison of the overall spindle length, and relative length of each of the 2 side's lengths

may be adequate. The Shimano UN series cartridge BBs are cheap enough that you are not out Much if you guess wrong and are ordering parts online

IF you Remove the existing BB and Bring it into the LBS, then the replacement will be less Guesswork..