How To Ask R U OK?
There are many different ways you can ask someone if they are okay, but start by letting them know you have noticed a change. R U OK? Can look like: "I haven't seen much of you lately, how are you going?" Or "You look a bit tired lately, how are things going?" Once you've asked the question, it's your turn to listen with an open mind. Don't rush them. Encourage them to explain and show you've listened by repeating back to them what they've said. You may then ask open ended questions, like "have you been feeling this way for a while?" etc.
Once you've listened, encourage action, to help lighten the load on the way they're feeling. This may be suggesting they see a doctor, or partake in a hobby that makes them happy. After this, it's super important to check in to see how they are doing frequently. This shows them you genuinely care, which can make all the difference.