How is the width of drop bars measured?
The most common measurement I've seen is c-c (centre to centre), the distance between the two horizontal tops of the drops, running parallel with the stem and behind the hoods.

If that doesn't make sense, see the following image.

Image credit bikerumour.com

However, you're right to check this as not everyone does it this way. The internal or external sizes might also be interesting if you're wondering how much space the bike will take to store or what size bar bag might fit between them.