How to check if a SMPS transformer is working properly
Checking SMPS transformers is always a bit hard:

You probably have no information to information that would tell you what the intended winding ratios are; and without, shorts between traces on either side are hard to detect.

Even worse, if it's e.g. a flyback configuration, it'd be non-trivial to reliably test isolation non-destructively, since these things tend to fail "OK-ish", until environmental parameters (e.g. humidity) change.

Also, since they are typically relatively custom components, it's usually questionable whether diagnosing the transformer is helpful: If that's broken, you'll have a hard time to find replacement, since you don't know the original specs. It's one of the things that typically lead to "oh, the supply is broken, replace it as whole". Also, in most SMPS, it's far more likely that semiconductor parts failed than the magnetics, so you're probably barking up the wrong tree.