  We demonstrate that gravitationally lensed quasars are easily recognized
using image subtraction methods as time variable sources that are spatially
extended. For Galactic latitudes |b|>20 deg, lensed quasars dominate the
population of spatially extended variable sources, although there is some
contamination from variable star pairs, variable star-quasar pairs and binary
quasars that can be easily controlled using other information in the survey
such as the object light curves and colors. This will allow planned large-scale
synoptic surveys to find lensed quasars almost down to their detection limits
without the need for extensive follow-up observations.

Astrophysics