How can a person determine if he is on the right side of history?

Whenever an argument occurs (hopefully a high quality debate), there are two groups of people, both with the same goal of “doing what’s right,” but both with different ideas as to what is “right.”

For example, consider the classic fight:
Conservatives VS Liberals
On the topic of the (fill-in-the-blank).

Have both people write up their respective platforms, and then ask the following question:
Do you believe that your platform is better than the other platform?

Both people will almost undoubtedly say “Yes.”

There can, however, be only one winner in the eyes of history. No matter what issue is at hand, eventually (far into the future) one opinion will be accepted by the majority of being “correct.”

This leads to the conclusion that simply believing an opinion is not enough to guarantee that you end up on the right side of history.

How can I know if I am on the right side of history?