If you're looking to get someone to tell the truth, but do not have any truth serum, you can try various other techniques to get it out of them. Learn some normal and drug free ways you can get the truth out of someone.
Act like you don't really care about finding out the truth. Someone might want you to react and beg the truth out of them. If you act nonchalant, they will want to tell you out of frustration.
Pretend you already know the truth and sound confident. Act as though you want to gossip with them, but need to know if they know the same truth you know about a situation or person.
Find proof of the story or truth. If you heard your friend stole some money from someone else and now has a brand new bike or Ipod, then you can present this situation to them. Most likely they will confess they stole the money as they won't have a story to back up the new gadget.
Use your own truth serum. Adults may give other adults alcohol to loosen their lips. People tend to tell the truth when they are intoxicated, so this option might be a good way to make someone tell the truth. You could try candy with kids, but its better if they tell the truth without this type of prodding.
Talk to the person's friends. Sometimes the friends will give you some information to go on and approach the person in question. If they do, you can bring it up and pretend you know the truth.