There are several ways of doing this.
First, you can split the fields, Lets say you have the combined vector up, containing the Mixed space for velocity and pressure. Then the first component can be obtained by:
u0=up.sub(0).sub(0,deepcopy=True)
Note that if you just save the velocity field to file:
u,p=up.split(deepcopy=True)
File("u.pvd")<<u
You can visualize the magnitude or each component in paraview.