You can't really know if they have helped, if they prevented symptoms from happening. Typical symptoms of gut flora imbalance are: diarrhea and/or constipation, bloating, gas, pain/cramps. Antibiotics typically target ALL bacteria, good and bad alike. How much/many are killed depends on a ton of factors- which antibiotic, how strong it is, how long it was used, what your flora was like before the treatment, your metabolism, what you eat/drink, how often you eat, your genetics and state of your immune system, etc.
You can't really get sick from probiotics (unless something is horribly wrong with that brand), as long as they are used at the indicated dosage. The strains selected are naturally-occurring "positive" bacteria that help to process/break-down food, and keep bad bacteria in check (by out-competing them, mostly).
Even if you don't take probiotics, the flora will repopulate, it just takes a lot longer. Not all the bacteria will be killed by the antibiotics, and you will ingest more of them from natural (unprocessed) foods (especially raw fruits and vegetables). The idea is to just accelerate the process quickly so as not to have the unpleasant experience (or shorten it considerably). You would normally take the probiotics both during and for at least a few weeks after the antibiotics are completed.
