Intel® Turbo Boost Technology 2.0: How to Enable and Use It
Max Turbo Frequency refers to the maximum single-core processor frequency that can be achieved with Intel Turbo Boost Technology. See www.intel.com/technology/turboboost/ for more information and applicability of this technology.
Other answers mention alternate methods to the basic CLI (Command Line Interface). I like to use Conky to do this. In the example below the Skylake CPU has a regular frequency from 800 MHz to 2600 Mhz. With turbo boost enabled the frequency can jump to 3500 MHz under heavy load.
intel turbo boost download 2.0
Now you can see the turbo boost is working by looking at the real-time CPU frequency. In the following screenshot, for example, the base speed of my i5 processor is 2.5GHz, but you can see the turbo boost is kicking in and giving more than 3GHz.