There is a more simpler solution to this:
you can check the option user has selected. E.g I have a group of 2 radio buttons containing value Yes and No.
I can check the value of the the same, using following SSJS code:
getComponent("radioGroup1").getValue()=="Yes". Similarly you can also validate it using getComponent("radioGroup1").getValue()== null or getComponent("radioGroup1").getValue()== "".
Hope this works for you.
Regards,
Pranesh