There is a mark on the outside of the tank that shows the proper level. Interestingly enough this info does not appear to be in the owner's manual. I found it in the service manual.
On a separate note, the service manual also states that the radiator side tanks are plastic, not brass:
The Crossfire has a cross flow type radiator (4) with plastic side tanks (3).
Plastic tanks, while stronger than brass, are subject to damage by impact, such as from tools or wrenches. Handle radiator with care.