None of my portable cases look similar, so it's quite easy for me to tell one unit from another.  I just remember what's in each case and where the case is stored.  Nice and easy.

Oh dear, Uwe, there's one in that pile that reminds me of my late lamented Alpina... <sob> Nice pile though! Some very familiar and others I don't recognise. 

I think I just kind of know what's in all my cases. Space is at such a premium here that I couldn't have a pile like this one, they are all either in a bookcase or on top of the book case or under my desk or under the bed I'm pretty adept at the storage solutions - if you didn't already know, you'd never guess there were 30 typewriters in my living room. But no, not labelled. I should do that...

 

Sitting side-by-side it can get confusing, although I can usually tell one brand's case from another (e.g., '30-'40 Royals and '50 Underwoods for example). To identify specific machines, I use a folded 3x5 index card attached through the case handle using a thin cable tie. I put the following info on the blank side of the card: manufacturer, model name, serial number, and other clarifying info such as color, year, or other distinguishing characteristics (e.g., a "rest" key on my Corona Model 3). For multiple examples of the same machine I add a #1, #2, etc. to the card. I also add a black dot to the upper corner of the card signifying that I have a photo of it in my collection photo album. And having just moved my collection from the U.S. East Coast to West Coast, labeling them certainly help me account for them when checking to see if any were damaged, thankfully none were.

I would love to have enough typewriters to need labeling.