I'd start by buying a few. Open one and let it decarbonate. It's must easier to discern flavors for uncarbonated drinks. As an example, Coca-cola has cinnamon

I'd start by buying a few. Open one and let it decarbonate. It's must easier to discern flavors for uncarbonated drinks. As an example, Coca-cola has cinnamon in it, which is almost impossible to taste when it's carbonated. It pops out when Coke goes flat. Most sodas will have a citrus component. Japan has odd ones like Yuzu, so try to pick up some essential citrus oils that aren't normal in your home country and are plentiful there. Then really, just put it onto stuff that you know the flavor of and taste/smell it to see if you can tell what's been added. Dip some white bread into it and see if anything comes out, etc. Don't be afraid to swirl it around in your mouth (like wine) or just breathe it in. FYI, I was just in Japan and realized that their Sprite has a lot more lime than other countries, which I didn't really like. Normally Sprite is great in hot weather, but the lime just didn't work in the heat IMO.