I'm going through a novel I finished two years ago, NICE GIRLS DON'T BITE. Good news is that I've gotten better at writing and can immediately spot and kick out the over-writing. Bad news? Although it was re-written many times back then, it still has spots of over-writing. Really good news? It still hangs together as a campy funny story.
I ask myself what writer fault is the most certain indication of amateurishness. I don't think it's grammar, spelling and the like. Those are merely indications of lack of education. Over-writing is certainly one of the top nominees, including its twin sibling, data dump. Perhaps lack of variety in sentence structure. And, oh my aunt Tilly, let's not forget unnecessary dialogue tags!