How to make sure shipper loads you properly?

Try to put the heaviest totes in the middle. If the weights arent marked, you'll just have to guess.
I've had that problem hauling Dal-Tile out of Texas. Ceramic tile is really heavy and the shippers have no idea how to place the pallets.

 

Not knowing how many totes you may pick up, and assuming the disparity between heaviest and lightest is not too great. An example of 19 totes might be
1
2
1
2
2
1
2
2
1
2
1
2

But that's the way I'd load 53 foot reefer suitable for California, but a dry van should not necessarily alter that configuration much. But if they all vary wildly and they are pre-weighed and tagged, I'd take the 8 heaviest ones and put 4 in first, and the remaining 4 in last, trying to keep the two lots about the same combined weight-wise

 

With scrap, even toted scrap every load is going to be different obviously. If they don't have scales on-site you are going to be screwed. Just keep scaling and taking it back to get it legal. After a few weeks you'll get a feel of which totes will need to be where.

 

That may be a worthwhile way of looking at it. But in my experience you want "adequate weight" at the rear. This gives enhanced teeter-toter weight shift effect when sliding trailer tandems.

Imagine having "no weight" towards the rear of the trailer. A 3 foot tandem slide will probably have negligible effect on the drive weight because there is insufficient weight behind the trailer "pivot point" to lighten the nose [if needed]

Plus from an engineering standpoint, I'd prefer not to test the integrity of the trailer support capacity by placing upwards of 2/3 of a heavy load inside the middle third of the trailer span. Especially given the way many bridge approaches and road dips are today that can cause severe bucking and testing of engineering limits of light van trailers.

Flatbeds are designed with this in mind, highcube 53 foot dry vans, not so much.

 

This is a crapshoot. 1 st find out how pallet sized totes there are in the shipment. Segregate them mentally heaviest to lightest. I would put 4 of the heaviest ones in the nose, spread out the lighter ones in the middle and the 4 remaining heavy ones right over the tandems. I wouldn't load any more than 48' of trailer, leaving 5' empty at the tail if it is a 53' trl. Tandems would wind up about 10' from the end of the trailer. Warn the shipper that if anything is out of balance or over, he will have to reload the whole thing. That might get their attention.