One great way to check your tenouchi is to look at the left side of your hand.
What angle is made at the wrist between your arm and hand?
I’ve always been told we should make our hands, wrists, and arms as one long straight bar but personally I have a tendency to make the back of my hand almost straight with my arm, making my tenouchi bend slightly inwards making it weak and causes my hand to flip at the wrist at the release.
Instead of this we need to make a Y shape with our hand so the bow can fit right in the center of the spot between out thumb and forefinger. When doing so, remember the angle that is made with the back of your hand, and protect that angle all the way from the time we make our grip through the release.