Tiffany:

Typically on 220 volt appliances, there will be three places to connect the three wires going to the appliance, and those three places will be in a recognizable "row". You always connect the white neutral wire to the middle location and the red and black power wires to the outside locations. It doesn't matter whether the black goes on this side or that, or the red for that matter, as long as the white is in the middle and the red and black are on either side, you're good.

If there is a location for a ground wire, it will not be on that recognizable "row", for if it were, it could cause confusion.

So, look at the terminal block on that heater where you connect the wires. If you see a recognizable "row" of three connection sites, it's almost certainly a 220 volt heater. Otherwise, it's a 120 VAC heater, in which case there will be only two connection points and possibly a place to connect a ground wire as well.

You won't hurt a 220 volt appliance by trying to drive it with only 120 VAC. All that will happen is that the heating elements won't get nearly as hot as they should.

So, maybe the safest bet is to try connecting 120 VAC to the heater and see if it works properly. If not, it's probably cuz it needs 220 VAC power.