I am not talking about whether I can submit a paper on subject A to a journal on subject B. 

My question is: 

How do I know *how good* my paper is? I could of course just send it to the best journal in the field, and see what they say, but I also don't want to embarrass myself and waste their time. If, however, it is very good and would be accepted by such a journal, it would be unwise to publish this in a lower ranked journal I would think.