How do I check if theorems I have discovered are original or not?

And please don’t tell me to submit to a journal because it is a terrible way of checking originality because journals can reject an article without check for originality, they also take a lot of time, and unless they find the entire article interesting they will reject it and will not answer your questions about originality. Also you might want to check the originality of just one theorem or maybe you want to check originality before submitting to a journal.