On one boat I had with an electric flush you would see and smell effluent coming out of the pump out. That boat had a small holding tank so I would leave the lid to the pump out a little loose because leakage there wasn't as bad as a leak or back up in the head. Of course there are gauges and on my Hunter you could look at the tank when the lazaret was open. The tank had some translucence and you could get a rough idea of the level.
But I used the reefing analogy. If you're wondering if it's time to reef - it is time. I would pump out early and often.
Holding tank level monitor, high level alarm, sound is also a good indicator, and the most noticeable method is the closer to full, the more it smells (even if you use chemicals) or when the toilet will not empty. You’ll want to dump for sure when you can’t take the smell any longer.