Cant make them they can always lie so its up to them to tell the truth and up to you yo believe it.
I wish I knew. But in my case: If it's someone u Love, tell them your feelings first. If it's someone u Hate, Demand that they speak and tell them u won't speak until they tell u.
Tell them how you feel! Often others wont open up about their feelings because in a way it makes them feel vulnerable. If you open up first it makes it easier for them to talk about their feelings because essentially what you have done is shown that you trust them not to hurt you by telling them your feelings so they feel safe in giving you that same trust. Even if what they have to say isn't what you wanted to hear its important that you don't become upset by what they say because how they feel is how they feel and while that may change you will look much more mature if you thank them for their honesty and trust. Then maybe things will change in the future but you havens ruined a friendship or potential relationship.
Hit them and make them mad.
Even if it is only truth at that moment.
It is still a truth of sorts.
Covert surveillance techniques are worth a shot.
Try hypnotize them? :P
It generally takes talking to get some one to open up......
You just sit and listen
Manipulation! Play the stupid game, ask questions and dance around it till they tell.
One has to be very subtle indeed. May be start off with asking leading questions.