I guess I'm kind of a "closet" fur too. None of my friends or family know, but I doubt they would care if they found out. The way I see it, being a furry shouldn't be equated to being gay in the sense that you need to "come out" to everyone. It's just something you're interested in. If someone asks, it can be an interesting conversation starter, but I generally don't bring my furry-ness into other parts of my life. Like Sabre said, if you don't make a big deal out of it, others won't either.
And as for finding out if someone else is a furry, I guess the only way to know for sure is to ask them, which might understandably be awkward. I suppose some people might make it more obvious by wearing tails or ears, or drawing anthros and the like, but there aren't many other specific signs. If you're desperate to talk to more furries, forums like these are great places to meet new people!