This is especially the case with celebrities. So while it won't help you figure out the pedigree of that guy in your History class, the website Jew or Not Jew can help answer the "Jewish?" question about celebrities. Sometimes irreverent or random, it's often very thorough, and it'll give you the low down on all the pedigrees, conversions, and misinformation. (I don't recall anything actually untznius, Not-Safe-For-Work, or offensive in the dozens of profiles I've read, but I could be wrong.)
They can be slow to add new celebrities (such as the ladies of the Black and Jewish video from Funny or Die), but there's a large database. You can get lost in there. And you'll keep coming back when you wonder, "Hmm...Jewish?"