Before I saw a Fox Sparrow, I spent a lot of time trying to figure out how to identify it, making sure I knew the field marks, etc., so I didn't mistake it for a song sparrow.
Turns out this was a bit of a wasted effort on my part-- after weeks of sorting through song sparrows asking myself "could this be a fox sparrow?" I finally saw one and it is unmistakable for a song sparrow. True, the basic form of it is similar, but it's much larger (probably at least 50% bigger) and while the song sparrow has a dominant white with brown & black mixed in, the fox sparrow, when seen from above, is much more brown. In short, if you see a bird which looks like a big, fat, brown song sparrow, you've probably got a fox sparrow in your sights.
As usual, the thumbnail links to a larger picture.