A while ago I looked for information on how Alexa, Wit.ai, Nuance Mix etc. do this intent classification and didn't find anything.

These guys have posted a nice blog post about their approach:

https://conversations.golastmile.com/do-it-yourself-nlp-for-...

They suggest that they add the word vectors in the sentence. But it seems to me that that would make the result independent of the order of words (i.e. "when does Tesco open?" and "Open Tesco when does" are the same). I thought I had tested that and it didn't work but actually I just tried saying "Tesco open does when?" to Alexa and it said "Sorry, I don't have the business hours for Tesco". Inconclusive I'd say but interesting anyway!