I have several very conservative, very religious friends. How would I test the waters and ask their opinions on homosexuality? I don't wanna come out only to be surrounded by people who don't want to understand.
You can be a supporter of LGBT rights without coming out yourself. So, maybe just be vocal about supporting marriage equality, and see what they have to say about that. Or, just bring up the subject as part of a political discussion or something.
I have many christian friends so I can tell you from experience you will get varied responses. I was really surprised that most of my friends were very respectful when I came out to them. If they are generally understanding and not judgmental they can keep their religious beliefs to themselves and still respect you. (one reaction) If they judge people a lot and talk about people's "sinful" actions behind their back this is a sign that they will not take it well, but if they can keep their opinions to themselves and keep an open mind you will be fine.
Like Ianthe said, bringing up a political issue works well. I've been staying in the closet around some people like that, but they know I'm a Libertarian and probably assume that's the reason I'm not against gay marriage (for example).