  Dark matter in the Universe consisting of macroscopic objects such as
primordial black holes may cause gravitational lensing of distant objects. The
magnification associated with lensing will lead to additional scatter in the
received flux from standard candles, and too small an observed scatter could
rule out compact dark matter entirely. In this letter, we show how the scatter
in fluxes of distant Type 1a supernovae could be used to distinguish between
models with and without lensing by macroscopic dark matter. The proposed SNAP
project, with $\sim 2400$ supernovae in the range $0.1\ls z\ls 1.7$, should be
able to identify models at 99.9% confidence, if systematic errors are
controlled. Note that this test is independent of any evolution of the mean
supernova luminosity with redshift. The variances of the current Supernova
Cosmology Project sample do not rule out compact lenses as dark matter:
formally they favour such a population, but the significance is low, and
removal of a single faint supernova from the sample reverses the conclusion.
