How can I tell if my Mac has Bluetooth 4.0 (Low Energy)

MacTracker provides detailed information for every mac ever made

http://mactracker.ca

If your mac has bluetooth 4.0, it supports Bluetooth Low Energy.

Here is a screenshot from Mactracker

For reference, Bluetooth 4.0 became standard on Mac Computers in mid 2011

Update

system_profiler also returns a plethora of information about the hardware. Run the following from the command line.

system_profiler | grep -m1 -i 'Bluetooth -- LE'

A Mac with BtLE will return the following

2014-01-15 22:26:46 -0700 kernel[0]: Bluetooth -- LE is supported - Disable LE meta event 

A Mac without BtLE will return nothing.

Update2

My answer is correct for using a GUI to find this information, MrWonderful's answer is better if you want to do this from the command line.

If you'd prefer a response regardless, try this:

(system_profiler -detailLevel full SPBluetoothDataType | grep "LMP Version")2>/dev/null

If it shows a version of '0x6' or greater, BT 4.0 is supported.

There is another similar post here.

Go to the "About this Mac", click on "System Report", look for "Bluetooth" under the hardware section and click on it. There is a line telling if BLE is supported or not.