yes, ask them yourself. (well no exactly because they could lie.
It's true that there are truth spells, but I feel you shouldn't force someone to do or say something (even if you had the skill or ability to cast one which I am not sure, you would only probably "suggest" persay that they tell the truth, it's still their choice).
Why not meditate on it? I find that opens my mind to little details, and you can try to look for hints.
Or... You could ask them