Is there a way to mass check addresses?
I've got 10k BTC addresses right now that i'd like to check since i'm working on something nice

the only way to do that is to write the code yourself because nobody needs such a thing so there is no easy way to do this. i did that for myself but just to check a handful of addresses (<10).
usually users need to check a couple of addresses at most so they only need to make a watch wallet on any wallet service provide like blockchain.info or even what nydiacaskey01 suggested.
on the other hand i don't think any of them would let you use their service for that huge number of addresses. because it would exhaust a lot of resources.