How to tell if my client is running a secure connection (SSL)

I am making a self-hosted app, and I would like to require HTTPS since sensitive informations might be sent. How can I tell if client is using a secure connection ?

• I could use javascript in the browser, but this wouldn't be secure (since an attacker could just bypass this)
• The node server might be running as HTTP, but behind a secure nginx/apache proxy.
• Optionnally, I would need to enforce this rule every time someone is making a request.

via Vinz243