How is brake fluid level detected?

Just over Min might be ok. There's really only one right time to check:

With the ignition OFF, sit in the car and stomp the brake pedal roughly 40 times. It probably won't be exactly 40, but you won't miss the way the last stomp really feels like a rock. At that point, you have cycled all the stored fluid out of the accumulator and back into the reservoir. That's when you should check the reservoir and, if it isn't right at Max, top it up.

As soon as you turn the ignition ON, the pump will run and some of that fluid will be packed into the accumulator. The reservoir won't be at Max any more, which is perfectly fine, as long as it's over Min.

If you make the mistake of topping off fluid while a bunch of it is 'hidden' in the accumulator, you've got a wet mess waiting to happen if the fluid in the accumulator gets cycled back later.

As far as I know, the level sensing does use a simple float in the reservoir somewhere. My reservoir has some interesting baffles right under the fill plug, so it might not be easy to see where the float is.

-Chap