How can I tell if two outlets are on different phases?
If there are two separate duplex receptacles there now 


Set your multimeter to measure AC volts.
Carefully stick one probe in the ungrounded (hot) side of one receptacle (the smaller slot).
Carefully stick the other probe in the ungrounded (hot) side of the other receptacle.


If you measure 240 volts (or there abouts), then the receptacles are on separate legs. If you measure 0 volts, they're on the same leg.

Keep in mind that the breaker and/or conductors that supply the circuit, could have been changed at the same time the receptacle was changed.  So you'll have to make sure the breaker and conductors are sized properly for the appliance you intend to install.

WARNING: If you don't feel comfortable jamming metal objects into an electrical outlet, you might want to contact a local licensed Electrician.