I guess we have to define what filler is here.
For me, filler is anything that doesn’t advance the premise or plot that was first established. Things that don’t have much importance to the plot, and things that are otherwise inconsequential.
to that end, it’s hard to determine if something is filler unless it’s different from the original author, such as maybe 200 plus episodes before a time skip or something, I dunno…
For the most part, there is canon filler, and non-canon filler. A lot of manga has canon filler that you won’t know is filler until the story ends, because you never know if the author will take some obscure reference from the beach episode and make it important.
Non-canon filler however, you can only tell if it’s in the anime but not manga.
I think the only media I find that has this is manga/anime. Western media doesn’t seem to have this issue… well okay… maybe that episode where Shaq was in an episode of static shock is probably non-canon filler since Shaq shouldn’t exist in the DC animated universe.