A litre of milk has about 1/4 cup of sugar, which is a similar amount to what you’d add to water kefir. The fizz and the growth of the grains (more visible with water kefir) are good indicators of how well the fermenting is going. You can taste to see if it’s tangy/sour as well. As for alcohol, I’m pretty sure milk kefir has more than water kefir, and using milk grains in water creates more than the sugary kind. I’ve just been reading Dom’s Kefir. I think you already know about it, so keep reading and you should learn a lot. I’ve only drunk about 4 batches so far.