How do you tell when your waste tanks are full?

When your toilet doesn't flush anymore, it's full.
When you have standing water in the shower pan, it's full. When you turn on the water faucet and nothing comes out, your fresh tank is empty.
Simple, accurate, works every time.

The light indicators are a good guideline. When the fresh tank is down to about 1/3 we start taking notice of the other tanks.