If your system is a base-board system you will have to open the door on the front of the end piece to find the valve. If the valence on the front of the radiator is too long you may have to pull off that piece as well. The valves on the base board systems are generally located where the pipe comes through the floor . The valve on a standing radiator is usually on the top at the end . Take note of the type of tool that will be necessary to open the bleed valve.
Some valves use valves that are opened with a straight-edge screwdriver.
Some use a square fitting that requires a radiator key . If your house has a second floor, start there. If there seems to be no room, a piece of air hose from a fish tank sometimes works. Remember, the objective is to get the air out, not run out huge amounts of water. Leave the valve open until you get only water.