Pro Tip™: if someone walks like a racist, talks like a racist, acts like a racist, they are, for all practical intents & purposes, a racist. You really do not need to know if they feel like a racist deep down in their heart of hearts to figure out what to do about them. This isn't rocket surgery, y'all. For a group of folks who coined the term "duck typing", we should be all over this.
To put it another geekly way: if someone does or says something racist, then follows it up with "I was just trolling, yo! I'm not really racist!" then one right response is "I voluntarily fail my Will save! I am thoroughly convinced that you are a shitbag racist, and henceforth will treat you like one! :D"
(Note: expanding on my twitter mini-screed here.)