Honestly, I think it comes with practice, at least for me.

I've been RPing for so long now that I'm able to remember what Caleb has heard IC and what he hasn't. You kind of get into a habit of it, eventually. When I was first starting out, I used the school chat as my source of OOC information. If it was posted there and I know that whatever situation was occurring and being discussed WAS DEFINITELY NOT witnessed by my character, it helped me to remember that my charrie didn't know about it. If that makes sense haha Same with "What in Merlin's Beard?" thread. I constantly checked that to see what was posted, and then most of the time, I GOT my characters involved so then I didn't HAVE to worry about OOC/IC!