Did You Know How to Tell if the Ref is Calling an Offensive Foul?

Did you know how to tell if the Ref is calling an offensive foul?

As many basketball games as I've played and watched, I just learned something new. I knew that if the Ref put his/her hand behind the head, that an offensive foul was being called. But I did not realize another way to tell is if the Ref has his/her fist closed.