Ok, wasn't sure of the question the way it was worded. Lightweight magnets do work to tell where filler starts and stops.

Places to look:

Many shops will leave evidence of filler at the bottom of the car as nobody likes sanding down low. Common to find leftover lumps of filler and/or heavier sanding scratches along the very bottom and under the rocker panel edge etc. that you don't notice unless you look for it. Sight down the sides of the car looking all the way to the bottom for any waviness and then use the magnets to verify.

Knowledge of where the type of car your looking at is likely to rust are good places to start. If patch panels have been welded in then some filler is likely. If it's been fixed right the amount should be minimal.

Flashlight and mirror to check down the sides of the trunk extensions at the quarter panels and up under where the rear window channel is in case the rear window channel has been repaired. Some people use fiberglass and other methods to repair rust holes around window openings for a quick sale.

Speaking of the quarter panels the sound deadender finish should match on both sides. If someone has been into the quarters doing metal work they usually don't spend a lot of time making the trunk finishes look oem again.