Besides loving them together how do delena fans know that they will be the end game and how do Stelena fans know that they will be the end game?

I'm certain DE will be endgame, like Elena said Damon doesn't think he's good enough to be loved. It wouldn't make sense to make Damon the most unlucky and unloved character on the show just to have him lose the only girl  that ever loved him back. They wouldn't give Stefan 3 women that truly loved him so he could end up with the one single women that ever loved Damon back. What kind of story would that be?

I agree with @CHOCOLADE.
It would be just wrong to leave Damon heartbroken, pushed away, unloved once again. It would be wrong to give DE this whole build up, amazing friendship, years of getting closer, Damon becoming a better man just to throw it all away at the end. It would be so wrong to have at the same time Stefan be trully loved by Kat, Rebekah & Elena, by basically every single woman he was ever interested in.
There is some kind of tv-magic that sais that when the bad, cynical hearbroken guy meets the girl who changes him, makes him believe again, they belong together. Like DE definately does.
The truth is, I do not know for sure they will be the endgame. But my faith is very, very strong. I just believe in it:).