The color of the body should be a slightly "muted" color if compared to non-dun horses of the same color. Of rather - it is oftenly slithly muted or a little diluted, but there are of course exceptions, and sometimes it's just a little difficult to see the difference. If you could see a very distinct true dorsal stripe and a little diluted coat, then it might be an indication of dun. The head is oftenly a little darker than the major part of the body, as well. To fully know if it's a dun and it has a lot of white, a test might be the way to go though.