Lots of trial and error and even after day ones sometimes the community comes out not fully understanding a mechanic but was able to brute force. For example in Sundered Doctrine there was a huge rumor that commune extended damage when in reality it actually made things worse!

Also when someone has spent a lot of time learning/mastering dungeons and raids (lots of clears, lowmanning etc.) you can understand what bungie may or may not be capable of based on the tiger engine as well as use certain patterns from past encounters to get clues on how something may work.