In my days, long ago, as an impoverished student when I loaded my own cassettes from bulk film, I have been known to put the spool into the cassette wrong-way-round, then spend a long time puzzling why it didn't fit the camera!
There's an old time projectionist's trick used to tell the emulsion side from the base.
Wet your lips and pinch the film between your lips. The emulsion will tend to stick to your moistened lips. The base side will not.
It's not really the recommended thing to do. It's not good for the film and it's probably not very good for you, either.
There's an alternate method. Wet your fingers and pinch the film between your moistened thumb and forefinger.
It works almost as well. If you do it on the tail of the film which is going to be cut off, anyway, you won't do any damage to the picture area.
I wouldn't rely on this trick all the time. I would chalk it up as old-timer's lore.
However, I have had a couple of situations where I used this trick because it was either too dark to see very well or where I had to find out quickly.
Sometimes the film will be partially threaded through a projector or camera and you'll want to quickly verify that it is threaded correctly. Just wet a finger and touch the tip or the very edge of the film. The tacky side will be the emulsion.