Sniffing an identifier for the iPhone app used to access a web page

When someone accesses a site via a WebView in an iPhone app, is there a reliable way (JavaScript or server-side) to tell what app is being used? It seems like the user agent string can tell me if it's from Safari or not, but not what precise app is being used if it's not Safari, unless a custom user agent has been specified by the app's programmer (which is not often).

I'd like to be able to tell more precisely what exact app is being used in order to serve [slightly] customised content. The identifier doesn't need to be human-readable, just 开发者_StackOverflowapp-sepcific (so e.g. a bundle ID would be fine).

Example (and not useful) user agents:

From Safari: "Mozilla/5.0 (iPhone; U; CPU iPhone OS 3_1_2 like Mac OS X; en-us) AppleWebKit/528.18 (KHTML, like Gecko) Version/4.0 Mobile/7D11 Safari/528.16"

Most Apps: "Mozilla/5.0 (iPhone; U; CPU iPhone OS 3_1_2 like Mac OS X; en-us) AppleWebKit/528.18 (KHTML, like Gecko) Mobile/7D11"

I tried to write just "Nope", but answers are required to be at least 15 characters long...