WHY would you need this info?
So, if you really need to know the version of css used, the answer is almost always css2, as css3 hasn't been got past the discussion stage.
@drjohn: First of all thanks for the info. And Why I want to know? Because I want to know.
OK currently CSS2 is used in most of the browsers but how would I now when my browser start supporting CSS3.
You will "now" when your browser starts supporting CSS3 because it will be listed in features or "whats new" when you get a browser update.
@macneato: Thanks for correcting the typo "now" :D.
Told you to take a look at Modernizr, link to the website is above. Think its jquery
Current browsers will not change their CSS support, it is only when new releases occur that they will start supporting CSS3. So when Firefox 4 or IE9 comes out, you will read about the new improved level of CSS3 support. But as CSS3 is still at the discussion stage, don't expect anything to change for several years. Just stick to using CSS2 and the odd bits that FF supports, and accept that without adding JS support for a few features, IE8 users (and those with even older versions) will not see the extras you add. It is not a problem in any way. Just don't imagine that your CSS2 designs will suddenly stop working - they will not.