For JSF:
//returns the major version (2.1)
FacesContext.class.getPackage().getImplementationVersion();
//returns the specification version (2.1)
Package.getPackage("com.sun.faces").getSpecificationVersion();
//returns the minor implementation version (2.1.x)
Package.getPackage("com.sun.faces").getImplementationVersion();
For Primefaces 3.x you can use the Constants
class in utils package:
import org.primefaces.util.Constants;
Constants.VERSION