  Due to the finite size of the disk and the temperature fluctuations producing
the variability, microlensing changes the actual time delays between images of
strongly lensed AGN on the $\sim$day(s) light-crossing time scale of the
emission region. This microlensing-induced time delay depends on the disk
model, primarily the disk size $R_\mathrm{disk}$ which has been found to be
larger than predicted by the thin-disk model. In this work, we propose that
light curves measured in different bands will give different time delays since
$R_\mathrm{disk}$ is a function of wavelength, and by measuring the time delay
differences between bands, one can 1) directly verify such an new effect; 2)
test the thin-disk model of quasars. For the second goal, our method can avoid
the potential inconsistency between multi-band light curves that may bias the
results by continuum reverberation mapping. We conduct a simulation based on a
PG 1115+080-like lensed quasar, calculating the theoretical distributions of
time delay differences between two bands: u and i centered around 354nm and
780nm, under and beyond the thin-disk model, respectively. Assuming the disk
size is twice larger than the standard one, we find that with a precision of 2
days in the time delay difference measurements, the microlensing time delay
effect can be verified with $\sim4$ measurements while with $\sim35$
measurements the standard model can be excluded. This approach could be
realized in the ongoing and upcoming multi-band wide-field surveys with
follow-up observations.
