You don't. Either you trust him and continue the relationship without the admission OR you give it up. It is almost abusive to ask someone a question and keep badgering them to give you the answer you think is right. And you can't really know if you weren't one of the two people involved--be very careful about considering what others say as the truth.
"Speak the truth" is often used to emphasize the act of verbalizing the truth, while "tell the truth" is more general and can refer to both speaking and non-verbal ways of expressing the truth. Saying "speak the truth" highlights the importance of openly communicating truthfully.
The advice "when in doubt tell the truth" is not a question, therefore it doesn't have an answer. I do agree that it is generally a good thing to tell the truth, and I endeavor to do so in the various answers that I post here.
To convince Hercules to leave on Mythology Island, you need to have the golden apple from Hades's tree. Offer it to Hercules as a gift and he will leave in search of more apples like it.
The statement "I always tell the truth" implies a contradiction because if the person always tells the truth, then they cannot lie, but if they were lying about always telling the truth, it would create a contradiction.
While honesty generally involves telling the truth, it is possible for a person to be honest but still not always tell the truth. Honesty is about being truthful and sincere in one's actions and intentions, but there may be situations where a person chooses not to disclose certain information or tells a white lie for various reasons.