How Do You Trick Some One Into Telling The Truth About A Lie?

Confess to something you havnt done which is slighty worse than what you suspect they have done. Record yourself on your phone telling them of your plan before you trick them and play it to them after they have confessed or revealed your suspicions were wrong.
However, if you love them then tell them about your suspicion and that you love them either way, and forget about it cause everyone has secrets.