The rear roll center is the height of the rear pivot ball - link or T-bar.
The front roll centre is effectively this. Draw a horizontal line parallel to the ground that goes through the centre of the lower pivot ball. For the right hand wheel, draw that line extending to the left. Now draw a line between the centre of the top ball, and the centre of the hinge pin, of the upper wishbone. This line will angle downwards. Where these two lines cross (somewhere to the left) join this point to the centre of the right hand tyre. Repeat for the left hand wheel.
Where the two lines from the intersection to the centre-line of the tyre cross (on the centreline of the car), is the instantaneous roll centre. Note that the roll centre moves side to side, and up and down, as the car rolls.
might explain it better than I did above! (Click on 'Suspension' and go to page three of that section for the diagrams.)
I said effectively. For suspension aficionados, this is not strictly correct due to the suspension design of the dynamic front ends on 12th cars. For any practical purpose, it works just fine. HTH