I do not think there is enough water in the system. How can I be sure?

The best way is to find a sight glass and check it. Not all systems have a sight glass. You can peck on the side of a tank and try to determine if it sounds full all the way to the top. You can open a joint near the top and see if it has water in it. Read the manual if you have one. If not, go the manufacturer's web site and see if you can find directions there.

If none of this works, post back with more details on the system and why you think it might be low.

I do not think ther is enough water in the system. How can I be sure? how do you know it not enough water?. cold spot?. a lot of noise?. ( hi velocity also make noise)... not enough heat output? Gurling noise??