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If you’re asking someone to open up to you, then you need to be upfront with them as well.  If you’re looking for honesty, you need to show you’re capable of reciprocating.  People are more likely to be direct with you if you’re direct with them. Start with a sentence like, "I want you to know that I feel____________about you/our relationship.  I'm hoping you'll be just as honest and share your feelings about me." Explain that you’re simply trying to gain some insight and clarity in your relationship, and that you respect their feelings no matter what.  It’s important for your partner to know that they can open up to you, without an abrasive reaction. Try saying something like, "I want you to know that I will completely honor and respect your answer, even if it's not the one I want to hear." There’s a chance you and your partner may be on two different wavelengths in terms of your relationship.  Their answer may sting a little.  Take some time to decide how you want to proceed in the relationship, knowing exactly how your partner feels about you.
Express your feelings first. Ask in a tactful, non-threatening way. Be prepared for the consequences.