The Speaker

Have you ever been on speaker when talking to someone on the phone and not know it?  It's awkward when someone finally says, "You know you're on speaker, right?"

Quick question: How would anyone know that they're on speaker when they haven't been told they're on speaker? 

I've spent some time this afternoon worrying about a few things I said on speaker. Wasn't horrible, but wasn't great either.  

From here on out I will start every phone call with the question, "Am I on speaker?"since some people think it's okay to put phone calls on speaker without telling you about it.