Any episode in which none of these apply:
+ There is a major event (i.e. a Gym battle or Pokemon Contest/Showcase/whatever), or introduction to a character (i.e. a Gym Leader) who will later partake in said event.
+ A new Pokemon is obtained by a main character.
+ One of the protagonist's existing Pokemon learns a new move or evolves, or otherwise develops in some meaningful way.
+ There is something relating to the Evil Team of whichever region our protagonists are in at the time
+ James crossdresses
+ A rival character appears
+ A significant character appears. This could be an Elite Four member, a recurring character who isn't a rival - like Professor Oak - or someone like Cheryl in the DPPt anime.
+ Special episodes, like the Ranger specials, etc.
...there might be more, but I find generally what constitutes filler in the anime are Character of the Day episodes, and none of those usually apply in those episodes.