The posh way is with a Romer laser system. It is slightly more accurate than a tape measure on a full vehicle.
If you can dismantle the suspension it is a lot easier. In the olden days for accurate stuff we used to use surface tables, measuring blocks, and big verniers, and a lot of 3D maths.
In practice when we are looking at competitor cars for vehicle dynamics that doesn't matter too much as the next stop is likely to be the K&C rig, from which readings you can reverse engineer the hardpoints sufficiently well. Alternatively you can skip the whole hardpoint thing and plug the K&C curves directly in programs like lapsim or carsim, which run faster than ADAMS and so are more suited to a lot of the work we do these days.