Stenzer you are rightly being conscientious. Many people would simply make a prototype, paint it with black matt paint, and then have a “Look” at it through the thermal camera to see how hot it was….in fact, many would simply touch it on full power and if it wasn’t too warm they would pass it.
In other words, people would not be as conscientious as you.
They might even make up the smps with the offtheshelf sample...and then find the efficiency...and they would then guesstimate how much of the loss was in the transformer...and then they would kind of make an experienced judgement call over whether it was ok.
Perhaps they would also make up a prototype......put it in a thermal chamber at max ambient, and at min vin and max looad...and see if the power supply survived 24 hrs in there and scuesfully produced correct vout and correct power...if it survivied, they would give it the thumbs up.
What you really need is a ferrite datasheet like the following DMR40 one…then you can look at the graphs on the second page…the top four ones. You can compare these with known ferrites that you have used before.
You really also should have a full transformer datasheet as in the attached. It tells everything from the wire used, to the exact ferrite material used etc etc
…but if you are going offtheshelf…you will never get this information…because if they gave it to you then you would go to a transformer manufacturer and get the transformers made for you and probably/possibly find out its cheaper than buying off the shelf….
So good luck…you can ask the manufacturer for the type of datasheets that I have given you here…but they will not give them to you , I am sure….sorry….as you know…its called Business unfortunately