Speak with Trial Master Rotun at the Temple of the White Tiger, then enter the Proving Grounds.
You there! It's <name>, right? Greetings. I'm Taijing.
<Taijing sizes you up.>
You look strong. I've seen you a few times here in the Vale, but I've never seen you fight before. I wonder... are you as strong as you seem?
If you're interested in proving yourself, speak with Trial Master Rotun at the Temple of the White Tiger, in Kun-Lai. He will put you to the test.