How is the width of drop bars measured? I'm looking to replace my current drop bars. I like the width just fine and would like to get a new set that is the same width (or at least close to it). 

How is the width measured? Is it the overall outer width? The width between the center of the bar ends? The width between the inner edges of the bar ends? The width of the flats up until the curve? Something completely different?
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.