You can’t ask just anyone about the condition of their soul. You will likely get either a “mind your own business” type of response or a blank look. You must have permission to ask personal questions like that. So if you care enough about a missionary to truly know the answer, you will have to earn the right to ask.
And that takes time.
If you have never asked such questions before they went to the mission field, don’t expect to start once they leave. If you have never been open and vulnerable with them, don’t expect them to be so with you. If you have not been around the block a few times together, don’t think a flight to the other side of the planet is going to make you closer.
Earning the right requires trust, and trust is not built overnight. If you haven’t had the opportunity to establish mutual trust before they deploy, be patient in developing it; it is much more difficult long-distance. But if you can, start before they leave your presence. Every missionary needs people in their life who have permission to ask the hard questions.