If their behaviour, actions or language are worrying you, the first thing to do is to make some time to talk with them. Once you have agreed on a time and a suitable place you should:

Tell – tell them you are worried about them and explain why in a non-accusatory way.
Ask – ask them directly if they have/haven’t had thoughts of suicide.
Listen – listen to their answers, don’t speak to fill silence or rush them to reply.
Keep safe – look at what you can do to keep them safe e.g.  take them to A&E, ensure they are in a safe location, stay with them till professional help can be accessed.