Correct method to trigger on mains AC signal

[canvas]

I am about to measure the interrupt time of an off-line UPS on power lost. I will measure its output mains voltage with my oscilloscope. I will be making the measurement through HV differential probe, so the polarity isn't a problem. I am expecting the AC sine wave will gradually deminish to 0V and later, the inverter will switch on. How can I trigger such event on an oscilloscope? Thanks.

[Andy Chee]

What is your oscilloscope model?

[ejeffrey]

You could trigger on the UPS startup then look at the pre trigger data.  One way would be to measure the current on the battery cable.