Recently, I was solving the problem of disabling a background video if the user was on a mobile device. Not only were the solutions very hacky, but they weren't even the right solutions. Instead of checking whether the device was a mobile, what I really wanted was to know whether the device prefers a low-bandwidth, low-battery usage version of the page.
It seems like this is a recurring problem: Don't send retina images for a low-res phone, Don't show HD videos on a low-bandwidth connection, Don't use highly-detailed textures while playing a game etc. Also, even for the same device, these preferences pay change with time - say when the mobile has a wi-fi connection and it can handle more bandwidth or the mobile is plugged-in so that battery usage is less of a concern while playing a WebGL-based game.
So, I think the browser should indicate such preferences to the server with each request. The browser has the best information at all times. Relying on User-Agent detection seems inappropriate because it remains the same in both the above cases - when the connection type or power status changes.
Is there any web standard, a request header perhaps, that indicates browser's preferences like the above? Are there plans to develop such a standard?
This video by Brad Frost, as well as his blog contain heaps of information on this topic.
The short answer to your question: no. But for further reference, in this question are a number of methods you could try.