The organism needn't know anything.
Testicular over-heating is one of the chief causes of infertility.
If creatures of a particular trait have more offspring than the others, soon, that model will dominate. And competition for resources will wipe out, or severely curtail, the disadvantaged.
Cold-blooded, sea-dwelling creatures didn't need too much of thermal regulation. Water tends to keep its temperature with great stability. Once things like laying eggs on land to avoid predation, and eventually living on land itself, brings their particular advantages, other things need to evolve as well- warm blood, for example. I'm not saying that creatures suddenly jumped out of water to develop warm-blooded bodies. No.
Species that laid eggs on land probably increased their numbers because of the increased safety for their eggs. These species probably differenciated further, with some of them living entirely on land with cold blood, and some others evolved rudimentary systems to keep blood thermally stable, say by means of mutated muscle tissue or benign tumour that always kept warm through breakdown of energy-providing chemicals.
The advantages and disadvantages of each model could further complicate themselves over the eons, with different sorts of adaptations through mutation.
One by-product of this gene mutation: Cancer.
I would argue that cancer is a primary driver, not just a by-product. This touches on a subject that hasn't been brought up yet on this thread - how genes can mutate/switch into different states. Some can become cancerous, others can switch from producing scales to feathers (eg dinos -> birds), etc.
And so we stumble along, slowly morphing into whatever is best adapted to the environment at any particular place/time.
You miss the point.
Evolution would make more heat resistent testes, not move them across the abdomen, through a hole that doesn’t exist for any other purpose and leave them dangling in a skin sack. That’s almost proof of sardonic creation.
BTW, birds have very high internal temps and their testes work just fine.