How to I tell coffee stains on teeth from actual tooth decay?

I have been keeping my teeth generally well as of late, yet suddenly one day I woke up and my two bottom teeth were brown in the middle and I have no idea how it got that way, so I kept brushing, the brown has faded but there is still remnants of it there, if it truly was tooth decay wouldn't it have started from the gumline and have shades of grey to it? They seemed fine last week, perhaps I had a strong coffee and it stuck or something.