Reliable method to detect mobiles

In my experience browscap.ini based checks are not terrible – at least. There is – probably not the only one – an PHP implementation with browsecap-php that can be used for PHP projects and would be suitable for WP projects too. As for a WordPress implementation, there is the plug-in – I know of – PHP Browser Detection, which as far as I can tell does a good job implementing the functionality. Last but not least, I can’t give you any numbers on reliability though, so lets just say, seeing the discussion under your question, you have been warned.