Dual layer discs DO indicate on labels what they are, this usually is a big DL, and as zion mentioned also feature the data capacity. Another easy indicator is that DL discs cost a lot more than single layer, often about six times the price. If you haven't seen any of the indications then it is quite possible your local media store simply doesn't stock DL discs as they aren't really that popular yet.

Again, as zion also said, only burners that specify the capability can burn DL media as it is fairly different to regular burning. Most burners produced in the last few months do, not many at all before that do.