How does it leave you feeling after you watch it?
You could think of this as being analogous to two things with food:
1. The aftertaste (eg smoked salmon leaves a gross smoke aftertaste if I eat too much of it)
2. How the food affects you later on (eg ice cream makes me feel sad the next day)
Example: After watching The Blair Witch Project I was terrified
Example: After watching Amelie I was love-sick for a week
Example: After watching Kill Bill I couldn't remember anything that had happened and felt like I'd just eaten junk food
Lindybeige - How to tell if a film is ART
https://www.youtube.com/watch?v=oCmrcym9Hro
Summary: if you were a film critic interviewing the director, and you asked "What is this film about?", and the only sensible answer is a summary of the plot, then that film is not art.