Simple test is to use a multimeter and measure against ground (a metallic waterpipe ideally but any conducting floor should give you at least a partial voltage reading). The neutral side should give you nothing. Of course you want to be careful when inserting a multimeter probe into a live electrical outlet! Note that touching the exposed metal of the probe while inserted in the live side of the outlet while grounded will give you a nasty shock.
320W sounds insufficient despite how they rate 25A at 12V. If it doesn't provide enough current you'll find that you can't get the bed and nozzle up to temperature in a reasonable time (if at all).
This may be helpful for you: AC power plugs on wikipedia. It sounds like the unpolarized outlets are a bit out of date. Generally the larger slot on a polarized plug is the larger one but the orientation of the outlet may vary, so your neutral may be the one on the left (or not if the installer put the outlet in upside-down).
320W sounds insufficient despite how they rate 25A at 12V. If it doesn't provide enough current you'll find that you can't get the bed and nozzle up to temperature in a reasonable time (if at all).
This may be helpful for you: AC power plugs on wikipedia. It sounds like the unpolarized outlets are a bit out of date. Generally the larger slot on a polarized plug is the larger one but the orientation of the outlet may vary, so your neutral may be the one on the left (or not if the installer put the outlet in upside-down).