Chiefly Hobbyists ............... and there won't be many per locality.
They'll do it empirically and by making use of pooled information.
And yes, they'll use Dishes - and Meters. Decent Meters can often register a signal that is below the level that a Receiver can resolve the signal at, so that helps enormously, in principle.
When 2F launched, tested and became operational, this very Forum ran several Topics for different areas of Europe (and beyond) to help collate reports and to try to create and plot the results to benefit others. That'll happen with 2E also. Other Forums and resources will no doubt do similar.
It's at such times that those with really big dishes are likely to clamber out of the woodwork - not now: They (and us) try to deal in real reports as speculation is nigh-on pointless.