If I assume that your system is a copper-tube finned baseboard system, then there may be elbows on the end of some of the radiation tubes under the cover that have vents. Depending on the type of vent, while the feed wsater valve is open, you may be able to open a vent with a screwdriver or a square-head key until water drips out.
Some installers do not put vents in the system and rely upon closing a balancing valve, then purging the air out of the system by allowng the pressure of the water from a connection to push air out of the whole system.
If we assume that the system uses radiators, then there may be venting valves at the top of the radiators.
You can see what the vents on the fin-tube radiation look like by going to http://www.hydronic.net to look at the hot water design pictures.
If there is doubt, a Licensed Master Plumber will arrange for air to be eliminated from the system and check the rest of the systm for safe operation.