Moshe as a young man goes out of the palace to see what is happening with his brethren in the outside world. He goes out one day and kills an Egyptian taskmaster. He goes out the next day and sees two Jews fighting. he approaches the aggressor and says. "Rasha Lama Takeh Re'acha" - evil one, why do you hit your friend!?
Rashi says he called him evil because someone who lifts a hand to hit another person is a rasha. The question is that the word "Re'acha" - your friend, indicates equality. in other words, he was saying the second guy is a rasha just like the first. What did the second guy do to deserve being called a rasha? How did Moshe know he was evil and deserving of such a title?
I suspect what happened was that Moshe knew he was a rasha because of the company he kept. They were obviously not fighting out of hatred each had for the other, as they immediately afterwords stood together accusing Moshe. I would even posit that they perhaps had staged the fight in order to lure Moshe in so they could show him what they knew.
Moshe could easily tell that they were in it together and scheming and conniving how to make trouble. he happened to witness only one of them raise his hand, but by virtue of association and context he was able to discern the second was equally evil.