You have to do some research. For things that are in print - Conjuring Archive is your best starting point. From there, you have to actually pull the books that have similar routines and see how similar the handling and presentation are. It's a lot harder with thing that are taught over video as there is no good index like the Conjuring Archive. As @Micahthemagician said, posting a performance on a forum is a good start.
Also, on a personal note, I find the safest procedure is to assume it's been published before - I know I've independently created several routines that had been in books I'd not read yet at least four or five times.