Sometimes, command words are hidden in the design
The DMG offers this on p. 136 (bolding added):
The identify spell is the fastest way to reveal an item's properties. Alternatively, a character can focus on one magic item during a short rest, while being in physical contact with the item. At the end of the rest, the character learns the item's properties, as well as how to use them. Potions are an exception; a little taste is enough to tell the taster what the potion does.
Sometimes a magic item carries a clue to its properties. The command word to activate a ring might be etched in tiny letters inside it, or a feathered design might suggest that it's a ring of feather falling.
So the third official option is to hide a clue on the item - you could etch a riddle where the answer is the command word. The benefit if this is that you do not need to have the identify spell, and you do not need to spend an hour of short rest to find out. And of course, it can be fun to discover it that way.
Identify in 5e can even unravel what artifacts do. If you don't like how easy it is to learn how to use an item, the DMG also has a variant rule called More difficult identification, where a short rest does not suffice (which is a nod to how it worked in older editions). In that case, identify should work, or you can demand both identify and experimentation to learn everything. If you play with this rule, and the party has no identify spell or you rule it also is not sufficient, their only recourse is a hidden clue, guessing it, or finding someone who knows the command word.