Look for the RAM door.

Check the bottom of your laptop for a small rectangular panel held in by a couple of screws. If you see one, that’s usually the RAM access door—you can unscrew it and see the slots. If the bottom is one smooth piece with no removable panels, the RAM is almost certainly soldered.

That said, some ultrabooks hide everything under the motherboard, so even a removable back doesn’t guarantee slots. But in practice, if there’s a separate door, you can swap RAM. If the whole bottom has to come off, it’s 50/50—look up your model’s service manual. Or just run wmic memorychip get formfactor in Command Prompt: a result of “12” means SODIMM (slot), “4” is soldered.

Bottom line: the panel trick works for most mid-range and older laptops. For thin-and-lights, assume soldered until proven otherwise.