1999 Forester follow-up question - how can I tell if there's enough coolant in the radiator?

When I take the radiator cap off, I can see coolant about an inch down, but
don't know how to tell if enough coolant is staying in the radiator.

For context, it seems that only some coolant is flowing back into the radiator
when the engine is cold.  But it also may be that the radiator was overfilled
with coolant.