Finding body filler

Best way to find body filler. And how much filler on a new painted car.

Evercoat Rage or similar can be found at any autobody supply place. One gallon can should be enough to do a car easy.

What the scrap guys use to check for steel-iron

First learned to use a piece of wax paper and then 2 or 3 magnets of varying strength. Start by checking the places you'd expect to find filler used by hacks. (rockers, blower doors, around windows, and then crumple zones. Use the weakest magnet first and then work up to strongest. Once you get the feel a car can be checked out pretty quickly.
It's also possible to "map" a car using a infrared thermometer
Good shops will use only a skim after body work. I believe the manufacturers of the product say no more than 1/4".
I'd prefer to see 1/8" max at thickest.

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.

I bought one of these and believe it or not, I have found it to be very accurate. How? by verifying its readings on cars that were being stripped to bare metal. The thing dead nuts on
I thought it was a K-tel type thing (junk for the younger members), but have been quite impressed with it's accuracy on multiple cars.

Filler Detector
I've had mine for several years and do store it with the batteries out as I do with all battery tools that are not used regularly

I'd shop on the pricing

If a shop can’t get a body straight with no SKIM COAT I’m going elsewhere.

Nothing wrong with using body filler the way it was made to be used.

Most of those laser straight high end paint jobs are built up first with filler, either wipe on or spray on, before being blocked down.

The clowns that use it inches thick or try to fill rust holes with it give it a bad name.

Over time it fall out

I use magnet backed dash plaques. Easy on paint and when they fail to stick I know there thick bondo. Usual places to find thick bondo is the rocker panels and lower quarter panels.

In my opinion, the straighter the car is, the more filler its going to have, albeit hopefully a skim coat. Save yourself some grief and don't bother looking unless the car looks like a mess and your looking for negotiation leverage.

AG.

Agree. Old cars that have had all the body gaps tightened up to perfection like a new BMW are done by shops that like to cover whole cars with filler and sand them down like you see on the TV shows. It's ok if they would sand the majority of it back off, but sometimes they don't so you end up with straight metal that still has filler stacked on it for no reason.