I believe this question has been posted to the 'obesity' community forum, so the first thing to say is that probiotic supplements are unlikely to have any meaningful impact on weight per se. There is a literature suggesting that variation in gut flora can influence weight, but that influence is very small compared to that of calories in, versus calories out!

If your interest is on general health benefits, then I'm not sure there is a good answer for you. Probiotics can have important clinical effects- we use them in my clinic routinely. But when we do, we can control product quality and dose. When probiotics are added to foods, organism viability and dosing become a matter of considerable guesswork.

With those caveats in mind, a few general principles:

-if you are going to get probiotics from food, the food should state it has active cultures, as yogurts typically do, and then the product should be kept in the refrigerator to preserve organism viability

-it should be a food you eat routinely- at least several times a week

-there are several useful strains of probiotic- acidophilus, bifidophilus, etc.- so no one strain is necessarily the 'best' choice

-to gauge whether or not you are benefiting, it may be best to take in probiotics for a 'reason' you can assess over time, such as a change in bloating or abdominal discomfort. Absent an outcome measure, it will be tough to say whether or not you are deriving a benefit. Not everyone needs probiotics to maintain a healthful balance of GI flora, although many people do.

Challenging question- I hope this is an at least somewhat helpful response.