I usually just de saturate then maybe a levels or curves to brighten. With practice it takes less time than typing this
Here's a great resource for retouching. Teeth whitening is explained here too and not very complicated. https://p-v.club/threads/the-retouching-series-w-pratik-naik.502/
I use the same method as Robert above. Creating the action yourself is super easy: 1) create a hue/saturation adjustment layer 2) desaturate -100 3) invert the mask 4) create a curves adjustment layer -> end action 5) brush over teeth with white brush 6) adjust curves as needed 7) adjust opacity to taste you can also use a hue/saturation and adjust the yellows, but desaturation and curves works well.
Just like the eyes use some moderation, most of the time pure white freaks people out with the whole uncanny valley thing
I found a free action online, it makes the teeth more white but its not what i whanted. I would try this technique on a 3D rendered image
sometimes I do use this on the sclera, once I get rid off the veins. But not too much or it will be unrealistic. Not on the shadowy part.