Identifying circuits is a simple, if somewhat time consuming task.
Multiple ways to do it, including physically tracing the circuits, toning them out, continuity test, and yes, energizing the circuits.
I probably wouldn’t bring a genny just for that, only because I have so many other smaller tools at my disposal. But any competent electrician should find this an easy task.
You should find one you trust, and then stay out of his way and let him do his thing, however he pleases to do it. He’s the professional in this field, and you are not.