Wait for the full picture before taking a side. The first version you hear is usually the most edited.

Whoever tells you first is not lying, but they are telling a story they are inside, with the parts that make them look bad already smoothed down. That is normal and everybody does it. The problem is that the first account sets your default, and every later fact gets judged against it rather than alongside it.

When you hear something that makes you want to pick a side, notice that you have heard one account. Hold the opinion loosely for a day. If it matters, ask the other person what happened, and ask it as a real question rather than as a charge.