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3T, e.g., measures c-to-c on the hoods. Deda, I think, measures outside-to-outside, but I can't remember whether that's in the drops or the hoods...
This seems to fit with my measuring. And yes, some sellers don't even tell you which doesn't help. Decent manufacturer websites always help but I'm currently looking at Nitto M109AA's and I know that Nitto don't even list them on their website...
Fox-
One of my pet peeves: manufacturers who can't even be arsed to just have a little archive section on their website that lists the specs of stuff they've made previously.
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The technical spec of 3T handlebars is set out with complete clarity on the vendor's website — as it always has been since 'the new 3T' emerged in 2008. It's a pity that superseded models are not available in an online archive but any vendor is understandably more interested in selling you a new product.
SwissChap
HarmanMogul
Often, sellers don't seem care enough. So I've usually seen it go like this: do the bars have packaging that states a number? If yes, that's how they will sell it. If not (i.e. used parts without a clear marking on it), they will measure it c-to-c. Hopefully. If you're lucky.
When it comes to figuring out which size you really want, there's no way past looking up how the manufacturers actually measure it. I've seen c-to-c / outside-to-outside at the bar ends, and c-to-c / outside-to-outside on the hoods. 3T, e.g., measures c-to-c on the hoods. Deda, I think, measures outside-to-outside, but I can't remember whether that's in the drops or the hoods...