How to pick a web application software engineer? [closed]

Question

How should I pick a web application software engineer?

The (permanent) position is to rewrite the client of an existing desktop client server application. The pages will not be generated dynamically on a server, but the server will expose a full API in whatever way is needed, for example JSON RPC calls, and can make static files available. It will replace a client that people have to download and install, so requiring a decent browser with reasonable settings (e.g. JavaScript enabled) is fine. Almost all use cases are for a desktop P.C. It will not be accessible to search engines (it is an enterprise application).

We can go so far as to write the whole thing in a single page, but don't have to. It would pretty much all be written by the web application software engineer in question. The exact open source libraries to use is also up to the engineer, within reason.

I'm specifically looking for advice on what qualifications to look for/verify in an interview, since picking a web application software engineer in general is just too broad a topic.

Edit - The position would be purely in-browser JavaScript programming (and be responsible for HTML and CSS) - other developers would develop the server but this position could request facades over the API. There is no Ruby, ASP, JSP, etc. because the web server layer is very thin and just translates calls to the business layer API and returns them as JSON (or whatever, but JSON seems easiest).