I've had a number of calls with prospective employers who, after I explain with them I want to transition to a product/tech-oriented role after some time with them, promise that I can make the transition. I understand this is probably just talk, but if an employer is serious, what are examples of evidence here?
How to verify?
You cannot verify that something will happen in the future. It is (still) a physical non-sense.
What can you do? You make sure it will happen in the future. Or, at least, you make sure to maximize the chance of it happening in the future.
For that, there is a very simple tool. It is called contract. In the contact it must be specified something like this:
Initially, OP will work XYZ. Between dates K and L, OP will transition to role EFG. OP will fully occupy role EFG starting with date M. The transitioning plan follows: (bla blah bleah).
If it is not written in the contract, the company will be free to do anything they want, and you will have no way to enforce that.
Even more, you might add tot eh contract a clause (if the company is professional and with good intentions) that they will pay some "penalties" if the said transition does not end properly, because of reasons you cannot control.