If you sew a lot, after a while you can hear the difference, and know your machine needs oil. Though if you're diligent with an oiling schedule, you may never get to that point.
Most people didn't sew as much as one might think - a few hours here or there, and oiled once a week or so, and it worked out OK.
The problem is we've since gone to so much "never service" type items that we've gotten out of practice. When I learned to sew (45 years ago) oiling was a normal part of the learning process. Same with oiling my bike, or the pony cart, or the lawn mower.
There have been so many times my husband is dealing with a machine, and the grease fittings are broke off or so cruded up it's obvious no one has greased the machine in ages. BTW - he keeps a grease gun in his car just in case he gets to a job site and they don't have one. He takes his pre-operations check list seriously, but he also has a heart for the machines he operates.