detect if user is on mobile device

[teez]

hello. 

I have button like

<a href='' id='' class='class A'></a>

and I need to change this class if user is using mobile version. What is the best way to do it?

 

<a href='' id='' class='if mobile then class A else class B'></a>

regards

[Sonya*]

Use css classes for Responsiveness