How do I know when my stomach is empty?

I'm sure this sounds like such a stupid question, but I have been given a medication and my Dr told me it is very important that I take it on an empty stomach and not eat for an hour after.

So how long after you eat is your stomach "empty"?

Also, anyone know why a medication would need to be taken on an empty stomach and why I can't eat for an hour? I have never been given a medicine like this, so I have no clue the reasoning. I did not even think to ask the Dr when she told me.