Quick tip if you can't tell: Look at its legs. A dun will have zebra markings, a buckskin won't. Easy as that. Haha.
My friend just got a red dun. As of today, actually! We've known him for 2 years though. Sorrels/Chestnuts aren't my favourite colour, but I loveee red duns.
My mare's a buckskin who does not have a dorsal stripe. I did meet someone who had the colours backwards and called her a dun because she DIDN'T have a stripe. I told him and he figured it out.