Red Hat / Fedora :: How To Verify The Root Password

I want to verify the root password. I am using rPath linux and my use case is like this: 1. There is screen in my application through which user can change the root user's password. He provides 2 information in the screen

i. existing password
ii. new password

2. I use a shell script in the back end that uses the "passwd" command to do it. And it doesn't ask for the existing password. But, my business use case is if user enters wrong existing password, I should not reset the password and throw some error message. How do I verify the existing root user's password? And also keep in mind that I am already in that linux box (logged in as root). Also let me now if there is any smarter way of doing it other than "passwd" command.