You can test soldered RAM in place.

Yes. Soldered RAM is still just RAM — you test it the same way you test any other stick.

Running MemTest86 or Windows Memory Diagnostic will tell you if the memory has errors, soldered or not. The difference is only in how you fix it. If soldered RAM fails, you’re not swapping a stick — you’re replacing the whole board or reballing the chips. But the test itself is unchanged.

If you suspect faulty RAM, boot from a USB stick with MemTest86 and let it run for a few passes. If errors show up, that’s your answer. No prying required.

Just don’t expect to fix it with a heat gun unless you know exactly what you’re doing.