There is absolutely no way that I know (native German) to tell without reading the label. Bottles, colors of caps or whatever, I have never noticed any pattern that discerns carbonated from not carbonated drinks.
non-exhaustive list of indicators that you have a carbonated drink:
- mit Kohlensäure
- (anything) Schorle (1)
- prickelnd / gespritzt (Austria, maybe southern Germany as well)
- sprudelnd / mit Sprudel (2)
non-exhaustive list that indicated a non carbonated drink:
- ohne Kohlensäure
- still / natürlich
- Saft / Nektar (if there are none of the positive indicators)
There is also "medium" used on mineral water to indicate that it is a little bit carbonated (i.e. less than normal mineral water).
(1) where I come from (northern Germany), "Schorle" always means carbonated. As comments on other answers indicate, in other parts of Germany it might not always mean that. On bottles, however, I have not once anywhere in Germany bought something called "Apfelschorle" and found it to be not carbonated.
(2) this is rarely written, mostly used when ordering in a restaurant, i.e. "Wasser mit Sprudel, bitte" - "carbonated water, please" - or you could be asked "mit oder ohne Sprudel?" - "with or without carbonation?"