Transformer Rating

If you can't trust the rating plate, what can you trust? Are you using calibrated equipment?

There isn't a field test to confirm the rating, other than to load it up to the rated value and see that it doesn't get hotter than the specified value with the specified cooling means under the specified ambient conditions. The main result of going higher than the rating is that the transformer becomes hotter (more current through the windings).

The manufacturers design the transformer to meet the MVA rating - they don't just build it and then test it to see if it works acceptably. They may do a heat run (if specified by the customer) to verify that the winding temp doesn't get higher than specifications during their in-factory testing.

Good Answer. But where do you find the right amount of extra load to connect it up to full load? What does a 1MVA resistor look like?

One or more big boxes filled with electric resistance heaters, plus a fan.

First he should become a field testing engineer obviously.

Get a job designing and building transformers.

Ok, first, get an education that qualifies you as a field test engineer, then you will know the answer to your question.

He loads it up, lets it soak for 24 hours, and records the temperature every hour. Otherwise he uses his mind to realize that better minds than his went into the design and manufacture of the transformer, and that the nameplate is the collective signature of approval of all those minds.

n.b. This assumes that the transformer is from a well known reputable manufacturer.

Needs to use a loadbank

That's the purpose of the test!

He or she reads the transformer factory test documentation provided with the transformer showing the pre-test results proving the transformer meets the design requirements.