The only way to know for sure to is connect it to a computer and see if it is registered as RVL-CNT-01-TR.
While the addition of the sync button on the battery cover is the common way to tell non-TR Wii Remote Pluses from -TR Wii Remote Pluses, even that is not 100% reliable. For this reason, this guide will not use the blanket 'Wii Remote Plus' term, but will use '-TR Wii Remote' to refer to the RVL-CNT-01-TR Wii Remotes specifically. Note that the very first Wii Remote Pluses, such as those included in FlingSmash and The Legend of Zelda: Skyward Sword, are not RVL-CNT-01-TR Wii Remotes. This guide tells you how to connect and use a RVL-CNT-01-TR Wii Remote Plus in Dolphin. Fortunately, they can work with Dolphin - with some special requirements. Since all Wii Remotes for sale in stores since 2011 are -TR Wii Remotes, this has become a growing problem. What exactly changed is unknown, but all -TR Wii Remotes may need additional hardware/software to work with Dolphin. Whereas the standard Wii Remotes and the very first Wii Remote Pluses register as RVL-CNT-01, the new Wii Remotes register as RVL-CNT-01-TR. Unfortunately, shortly after the Wii Remote Pluses came out, a new Wii Remote Plus type appeared. Released only a year after the Wii MotionPlus accessory, the Wii Remote Plus is a Wii Remote with the MotionPlus addon built into the Wii Remote.
