How do I know that my yogurt has live and active cultures in it? I used store bought organic Greek yogurt with live and active cultures as my starter, and it turned out beautiful and thick,but I'm making it to replace taking probiotic capsules. In all my studying beforehand I never found a conclusive answer as to, HOW do I know if the yogurt bacteria is in it?
In the US yogurt will list in the ingredients the particular strain(s) of lactobacilii that it includes. It will also say on the label: "Contains live, active cultures"
Even the cheap yogurt I have looked at says it contains live cultures.
The fact that you made the yogurt is proof enough. Your yogurt has live cultures.
In principle every yogurt is a colony of bacteria living in milk. When you make new yogurt, you are establishing a new colony, using the starter as "colonists" who then take over the new milk. The result is a lively ecosystem of bacteria.
Companies nowadays optimize food for long storage time, and frequently also for some blandness. Since a lively colony can go more sour with time, some of them actively go and kill the bacteria after the yogurt has been made. This stops the yogurt's taste from changing.
The reason why people warn you about choosing "live cultures" for homemade yogurt is that, if you happen to pick a storebought yogurt with killed-off culutres, they won't be able to colonize your new milk. Instead of yogurt, you will end up with spoiled milk, wasting time and food. They don't mean that you will make somehow "dead yogurt" from a dead culture.
If it works, meaning that with two tbsp of the store-bought yogurt you can make a new 1lt batch of yogurt, well, it has live culture!
Anyway try not to keep the new made yogurt in incubation for too long, because the longer the time you leave it, the more the bacteria thrive, up to the point that they eat all the lactose and finally die due to overpopulation! normally 10-14 hours are enough to make a thick nice yogurt, but avoid longer incubation times if you prefer a milder yogurt. One of the problem with storebought is that you can keep using the yogurt to make fresh batches just for 3-4 times, after that it will no longer work and you should buy another "mother". If you are interested there are "heirloom" cultures, that instead you can perpetuate indefinitely. (I have been making mine from the same mother from 5 years now). Many people also are keeping their yogurt for years having started from store-bought.