The power of a specific martial arts? You go to your Martial Arts tab and click on that martial art to see what its current Power is.
You can check the manual to see what the power would be at level 10 (Max).

There is nothing measuring the overall power of your character compared to someone else. You can more or less tell if you long press on them in combat, or use the Info button when interacting with them to see their stats and see how they compare to yours.

ok thanks