Warranty is not transferable. You'll want to check along the edges for any tool marks. If it's been opened and had anything replaced, unless they're better than 99% of the DIY steam deck crowd, they left small traces of getting past the backplate.
Watch a video or two on how to replace the SSD, and go looking for marks around the areas people said are easier to slide things into to pry the plate off. If you want to open it yourself, you can also check the screws for any marks as well as ensure there's no glaring issues with the internals in case they tried to replace the joysticks or something. Valve was pretty clean with mine, every screw pretty much pristine.
No, yeah, the opening of the device is more so a "did they replace/damage anything" and less of a warranty problem. Second hand sale is what stopped warranty