How to check the battery level of devices connected via Bluetooth in OS X
I hate to have to answer in the negative but the answer is: you can't. 

I've done a lot of searching the web, testing utilities, playing with ioreg on the commandline, scrutinizing System Profile reports, and it appears to me that OS X has no way of displaying battery level of connected devices that don't automatically display battery status in their Bluetooth menu Device submenu, even if iOS devices display detailed level info for the same Bluetooth device when connected to them. OS X just appears to completely ignore this capability in all but a few default (Apple-manufactured, it appears to me) devices.