If by "find the line" you mean you want to know what the incoming power feed is to this box, it appears to be the left most cable coming into the box. Its black wire goes to the yellow wirenut feeding more black wires going tot he switches. Its black wire goes to a similar gang of white wires tucked back into the box there with the red wire nut.

The other cables seem to be going to individual switches (they're the wires going out to the fixtures). The right hand switch looks like it has more wires. Some of the automation devices require a connection to the grounded (neutral/white) conductor.

Voltage is always a difference, so yes you must connect one probe on the voltmeter to one wire and the other probe to whatever you want to use as a reference (ground works, so will a grounded conductor or even another hot conductor provided you understand what it is you're measuring).

Thanks.

So for the new dimmers that are not 3 way (old are 2 leftmost in the pics) I need to use the white to goto the pigtailed white wires (assuming they are neutral)? Recall the Insteon switch has line, load, neutral wires with ground