Someone has bloody poo how do I find out?

We have 4 cats and they all use the same litter boxes except for Ivy.
How are we supposed to rule out who to take to the vet?

The eternal question.

First rule out the suspects. I know my cats poo based on size and consistency, and often by smell. Is it a BIG poo? Then rule out the kitty who does little poos.

Also go by your gut. Who do you feel it could be? I've never been wrong by following my instinct.

Yeah, I kinda know my cats' poop, too. How pathetic is that. :p If you don't know who does what, you'll probably have to isolate one, then another until you find out who has the bloody poop. Good luck.