Well, we learn them as children, so it feels natural ;) Also, there are 'questions' that we ask ourselves sometimes to figure out the case/the form (yeah, we do need it sometimes), but I don't think that's helpful to a non-native, actually.
Anyway, here it would be: jestem w (czym?) (I am in what?) -> w domu, miejscownik (Locative).
Yes, i ask my girlfriend if a certain sentence makes sense and obviously she will say no because im rubbish ;) but then i ask why, and she just says that it doesn't look right which annoys me because i have to think so much about each individual word and the case to apply. Thank you though for all your help