there is a tan-stripe morph I'm not entirely sure how you distinguish them from juveniles who also have tan-striped crowns – possibly their throats are whiter than the juv'shere is one source showing photos of the white-striped as well as tan-striped adultshttp://www.allaboutbirds.org/guide/Whit ... Sparrow/id
The first way too tell them apart is to know when not to bother - there are no juvs in the spring. Later in the year, the juvs have heavier breast streaking and less distinct facial patterns.