You have to search segment-by-segment on the new site just like on the old site; that has not changed.

Beyond that, if you're using Chrome I've got an ExpertMode+ browser extension which can help call out the flights with availability.

http://blog.wandr.me/2015/08/united-...-new-features/
n.b. The link above is to my blog or to one which I am a regular contributor. FT rules require that I disclose that in the post.