To prejudge is to make a decision about something before you have all the facts. If you prejudge a game, you decide who's going to win before it starts. Better hold off on the victory party until it's over. When you judge someone or something, you "form an opinion or a conclusion" about it.

What is the word for judging before knowing?

Prejudging means to judge someone/something before knowing or having enough information (the prefix pre- also indicates that).

When would you prejudge a situation?

If you prejudge a situation, you form an opinion about it before you know all the facts. They tried to prejudge the commission's findings.