Using geometricJacobian and velocities

I used the geometricJacobian method that generated a the  matrix for a puma. So I underatand v = J theta_dot, where v is a vector of cartesian velocities, and theta_dot is is a vector of joint velocities. When I created the rigidbodytree and rigidbody, I did not specify any velocities. Am I supposed to spectify velocities when I create the rigidbody joints? With the Jacobian I got from using geometricJacobian, is there a method that returns the joint velocities and the cartesian velocities, so I know what they look like, and I can multiply the J with the jount velocities, to verify the result of the cartesian velocities? I am confused where the velocities are basically, and which functions are there to obtaon velocities.