How to verify if all 3 AC phases are in order?
You get a phase sequence indicator. There are two kinds, one uses a small 3-phase motor and it rotates in one direction or the other. The second kind does it electronically. They start at well under $100. 

You should not attempt to connect an oscilloscope or an electronic grade DMM to the mains, especially 3-phase mains, unless you have the proper equipment, including probes and you know exactly what you are doing, you could be injured, die or destroy your equipment. Not only are the voltages high, but the potential fault currents tend to be extraordinarily high (I've seen as much as approaching 10kA in a light industrial situation).