well, mine is carbonated and loses the sweet flavor the longer it sets (it can taste more like alcohol as it stays out). when i make grape soda, it's just taste carbonated & somewhat sweet (but i ferment it twice, adding the grape juice the second time without the grains)... but if i make it with just lemon and molasses & raisins...it taste more like a long island iced tea kind of (best thing i can think to compare it too??). you will know if it's fermenting if the flavor just isn't changing whatsoever. taste it when you start to make it & then taste it again in 24 hours. is it different?
wait...actually, a better idea is to take the grains and put them in a jar of spring water and add molasses and grade b (or c) syrup. my grains will triple in a few days when i do this. i only do this to grow them though, i don't drink the stuff- but they will grow like mad. just mark the glass with a marker and see if they've grown within 48 hours....that should tell you for sure. i hope this helps.