The simplest setup....which is best for most users....is to use the default setup where both the 2.4 GHz and 5 GHz bands use the same wireless network name.
With this setting, wireless devices will automatically choose the best quality signal with which to connect based on their capabilities and distance/location in relation to the AirPort Express.
If both are available, will the device always choose the 5 over the 2.4 (if it can handle 5)?
No. 5 GHz signals are much weaker than 2.4 GHz signals, so they do not pass through walls, ceilings or other obstructions nearly as well as the stronger 2.4 GHz signals.
If you have a device that is capable of connecting to 5 GHz, it will do so if it is in close proximity....the same room or it has line-of-sight with the AirPort Express. But, that same device will likely connect using 2.4 GHz if it is a room or two away from the AirPort Express since the 2.4 GHz signal will be much stronger than 5 GHz at that location.
If you use the option to assign a separate name to the 5 GHz network, in theory, you can "force" a device to connect to the 5 GHz band. This might make make sense if all of your 5 GHz capable devices will be in close proximity to the AirPort Express.
But, connecting to 5 GHz is no guarantee of a better or faster signal if the device is located some distance away from the AirPort Express....where the 2.4 GHz signal might be not only stronger, but faster than the 5 GHz signal.
So, to review again....If you leave the AirPort Express in the default setup, devices will automatically connect to the band with the best signal quality based on their capabilities and distance/location in relation to the AirPort Express. Isn't that what you really want?
How do I know whether the Apple TV (or other device) is using the 5ghz or the 2.4?
Use AirPort Utility to check the connection details for each wireless device.
Hold down the option key on your Mac while you double click the AirPort Express icon in AirPort Utility. Each wireless client will be displayed. Click the arrow next to each wireless client for more connection details.