Finding related result in mathematics
You're asking for the impossible.  Almost every mathematician with more than a few papers has had the experience of submitting a paper or circulating it as a preprint and having some relevant work they didn't know about (and are not expected to have known about) pointed out to them.
If you are asking about one or two specific narrow questions or equations/formulas, then usually the best thing to do is to figure out who the friendliest expert who might know is, and ask them.  Even if they don't know, they might refer you to another friendly expert.  (Some subsubsubfields have friendlier experts than others, and this difference definitely influences the popularity of the subsubsubfield.)
If you're looking to break into a new subsubsubfield with lots of history without an advisor, then some subsubsubfields have friendly conferences, but, in general, ...uhh... good luck.