How to test which version of TLS my .NET client is using?

If you turn on “CONNECTS” in Fiddler, you can see the TLS/SSL version in Inspectors -> TextView

To turn on Connects, go to Rules in the menu bar and remove the check from “Hide CONNECTs”

Note: Decrypt HTTPs traffic must be disabled

Reference: Viewing HTTPS Handshakes in Fiddler