How to measure confidence score of Speech Recognition

Myself saikumar, I am using this plugin in my application and it working fine,it’s giving the output what i am speaking along with matched record.
Now i want to know "How to measure confidence score of Speech recognition plugin means how much accurate this plugin recognizing the voice input from user "
It will be useful for me to implement this plugin into my project

Is it a paid a one or a free because as u as mentioned in a plugin description It uses Speech API of Google and it is a paid one after some hits can u a clarity about it