This can be done in two ways:
First method:
Plot concentration/dose versus time for all subjects and look at the descending slope. The descending slope should overlay each other for the subjects and be linear in time.
Second method:
The second method is accomplished via non-compartmental analysis (NCA) for each subject. The lambda Z values versus dose should be plotted and there should be a horizontal line. This is also true for the AUC versus dose where AUC should be relatively constant for each of the doses.