The lactose sugar disappears as the yogurt is fermented, IF it’s fermented correctly meaning for the correct length of time and at the right temperature. Any brand can contain a bad batch, but the batches are supposed to be tested before packaged and sold. Still, no guarantees if it’s commercially made.
You should be sure to look for plain Greek yogurt or unsweetened. Some dependable Greek yogurt brands are below:
The Greek God
Taste of Inspirations
I’ve never tried the brands that Benc named, but that doesn’t mean they’re not suitable.
If all aspects (temp, etc.) are right for the test, you can determine if the bacteria in your yogurt or kefir are alive and active by putting the yogurt or kefir into a clean glass, cover the glass tightly with plastic wrap and leave the glass overnight in a warm place such as near a warm heater, but not too hot. Come morning, if the wrap remains flat, it means the fermentation didn’t start and the gases didn’t form because the bacteria were not alive or active.