Expension tank and radiator cap

I never noticed the sight glass before...  I guess if you don't see coolant there you know you are way too low.  But you are supposed to have coolant right to the top of the expansion tank.

I usually just have a look at what's in the overflow tank, to see if I'm losing any fluid between flights.  There's a reason why it's transparent, and why it has MIN and MAX markings.  After a flight it will contain more fluid than when cold, but that "extra" will be sucked back into the expansion tank as the engine cools.

My installation does not use the Official Rotax Expansion Tank, so I have no sight glass.
My overflow bottle and the hose going to it are transparent.
As long as the Bottle is half full AND the hose is completely full, I know the Expansion Tank is also completely full.

The sight glass would also be a good indication that the expansion tank is full.
There no reason to remove the pressure cap before each flight to check.
If your sight glass shows full, and the overflow tank has fluid, you're good to go.
If your connecting hose is transparent and shows full, even better!

Interesting idea of having a transparent hose to the overflow bottle.  Mine isn't.  I guess I just figured if the amount in the overflow goes up when it's hot and back down when it's cool, it implies the expansion tank is full and that connection is working properly.

If you take that cap off before every flight you'll be buying $80 caps more than most of us.