Your fuse box will hopefully have written on each fuse the amount of amps per fuse, usually 15, 20, or 30. One of those fuses will be connected to circuit in the room in question and hopefully labelled.

Meanwhile for the heater, figure out how many watts it uses then convert to amps. For instance at its highest setting, my heater is 2400 watts. Plugged into a 240 volt PowerPoint it uses up 10 amps