Simply ask them outright, from a position of empathy

If all of these routes seem counter-intuitive to you, you may just be better asking your partner point blank if there is something they would like to talk about and if there is anything you can do to help them if they are unhappy. 

This can work wonders because it will show them that you’re intuitive, care enough to address their issues, and will get down to the point. 
“I suggest not beating around the bush, and simply asking them outright, from a position of empathy and concern versus hostility,” David Bennett, counselor and co-author of the site “The Popular Man,” told INSIDER. “Many relationship problems develop because there isn’t direct communication.” 

This approach does come with some risks though. It can be jarring to point out possible unhappiness out of the blue, so make sure you approach it with care. Don’t reach for the question in a moment of frustration, but do it in a way that let’s them know that you see how they might be hurting and that you want to help. 

Psychotherapist Gary Brown suggests a question like this:
“I’ve noticed that you have been a bit withdrawn lately.  I care about you and am concerned.  Can you tell me what’s happening for you?”

https://www.independent.co.uk/life-style/relationships-psychology-partner-unhappy-three-things-to-say-honest-direct-a7837266.html