I'm at the point in my textbook where it explains the use of -ている and the three kind of verbs related to its use: stative, durative, instantaneous. The book says I should ask myself if saying something such as 昨日は１時間買っている is logical. At first view, it seems it isn't, and 買う is an instantaneous verb (buying something, which is exchanging money for something, over an hour, seems a bit ridiculous). But thinking about it a bit more, this situation came to my mind:
In this context, it does seem possible he's saying 'I'm buying food'. So, my question is: is there a fool-proof way to know if a verb is durative or instantaneous? Some list or tool around I can use to verify what type of verb each is?