The measurement of the optical phase of far-detuned laser light by phase mo
dulation spectroscopy is promising as a non-destructive probe of the dynami
cs of Bose-Einstein condensates. We have developed the experimental techniq
ue on a room-temperature Cs vapour cell, and a Cs magneto-optical trap (MOT
). We measure column densities with an accuracy better than 7%. A numerical
model, including Doppler broadening and transitions between all relevant h
yperfine levels, accurately describes the data from the Cs cell. The same m
odel without Doppler broadening gives quantitative agreement with measureme
nts from the Cs RIOT. We show that the conditions needed for non-destructiv
e measurements of BEC dynamics are achievable with conventional equipment a
nd a simple, robust experimental setup.