I'm still working my way through James Loewen's book about Sundown Towns (he's talking about how and why the information about a town's status as a sundown town was often later concealed). Fascinating stuff.
You can visit his web site for a list of American sundown towns, both suspected and confirmed, along with some info about their history. Just click on your state in the map and see if any of the towns on the list look familiar.
It's hard to knw if all his information is super accurate. He lists Brookline, where I live now, as"probably not" and I'd say it's definitely not a current sundown down (not that we don't have racial problems). And I'm not certain that it ever was, though that could just be denial--I'll have to look in the history books. After what I've been reading, I guess it won't surprise me. The Colorado list is much too short. Still, the site is worth a look.
Another town where I've lived that is or was a sundown towns is Mahomet, IL It saddens me to think of Mahomet was sundown, but I suppose it shouldn't completely surprise me. When we lived in Champaign, we heard rumors that Arcola still is (and it shows up on Loewen's list as probable.)