Who says that you have two-phase power?

This is the same person as the "lutron dimmers going crazy" which Lutron blames on the common neutral in a MWBC.

The "test" an electrician will do is to look at the wiring for the circuit (typically at the breaker panel). A MWBC will be obvious (there will be two poles feeding a cable with only one neutral).

Besides a correctly wired MWBC with a two pole breaker feeding a cable with one neutral, you could also get an accidental / illegal MWBC.

Consider a multi-gang switch box with multiple circuits feeding into it. It would be easy enough for somebody to pigtail all those neutrals together. It could be this way for a long time and nothing will seem wrong (other than current being carried on the 'wrong' neutral).

Two clasirify two-pole breaker is not strictly required for a MWBC.
What I meant to say that it is being fed by two different poles (which might be a two-pole breaker or it may be two single poles).

JReagan is correct, someone may have indeed done that, but frankly I think Lutron's excuses are chickenpoop.

Duplicate thread. Go here: http://ths.gardenweb.com/forums/load/wiring/msg0116545813876.html?7

"Consider a multi-gang switch box with multiple circuits feeding into it. It would be easy enough for somebody to pigtail all those neutrals together. It could be this way for a long time and nothing will seem wrong (other than current being carried on the 'wrong' neutral)."

This is an issue that I see frequently, and there are still electricians out there that think it's okay to tie all the neutrals together in every box. For example a 3 way that is set up with the line in one box and the load in the other should have that neutral coming from the line side switch connected only to the load, even if the load side box is on the same circuit. If you bundle them all together you now have parallel paths for return current. I see 3 ways done with the line fed into one switch and only a two wire ran for travelers and the neutral picked up at the load side switch box.