if your switch isn't saying there's an update available upon launching the title (my switch knew that undertale, l.a. noire, and a couple of other games needed updates despite not being connected to the internet in a literal year), you'll probably just have to compare your version number with whatever information is available online. you can press + on the game's icon on the home menu and it will show you the version number at the top beneath the title.
just now i searched "undertale switch update" and the top result was "version 1.11 now available". so if your version is 1.0....obviously there's an update. i'm sure there must be a database somewhere out there that compiles current switch updates in a more accessible manner, but i haven't searched for it.
edit: since you're superbanned and probably will have to obtain your updates from......dubious sources, it's likely that someone on the site you'll go to has a post of just switch updates, or includes updates with their game listings.