As Ian wrote, Do
NOT include the trump King in your response to a 5NT "King ask." You've already shown it--do not bid it twice.
The most common methods are the two Ian mentioned.
1. Number of Kings
2. Specific Kings -- this is a superior method to #1 above.
A third method, one not so widely known, is the
Denial Method, in which the bidder responding to the 5NT King-ask bids the lowest non-trump suit in which he does
not have a King. I have not played this, only read about it. I first saw it played on Vugraph by Gavin Wolpert. A recipe for disaster if not in synch with partner!
Regardless of which method you use, most people (except Kit Woolsey) who play RKCB use 5NT (or its equivalent if playing Kickback) to show all five keycards. Knowing this, if the partner of the 5NT bidder has an undisclosed source of tricks--or some other feature that allows him to count 13 tricks between the partnership--he should just bid the grand rather than showing his kings.