IMO, if a song writer composes 100 say songs then generally he/she might get a couple of quality ones that will rise above mediocrity.
Sometimes a composition you think is a good song may not take off while others you may regard as a filler may have that something extra that others hear.
That said, I think if you ever write a real cracker (along the lines of 'Bridge Over Troubled Water'), you will know. :-)