Figuring out what's up with your teenager. 

Ask them to answer this question: 'How are you on a scale of 1 to 10?' 1 is pretty awful and 10 is smiles all round

If they answered 7 or above, awesome! Ask them what's going well in their world and remind them of these things the next tmie their score is low. 

If they answered 4 or lower, ask them what might need to happen to bring them up to a 7. Check out reachout.com together for some advice. 

Ask regularly. It'll help you spot trends and act quickly. 

Tips for creating a safe and trusting conversation:

Let them speak. Give them time and space to talk, and avoid interrupting them. 
Remove distractions. Put away your phone and give them your full attention. 
Acknowledge their feelings. Respond with comments like 'Tha tmust have been hard' or 'Sounds like it was exciting'
Give physical cues. Nodding, making eye contact and facing the person you're talking with show that you're really listening. 
Plan for later. Ask them if they'd like to chat about the issue again, and lock it in.