  The binary fraction of a stellar population can have pronounced effects on
its properties, and in particular the number counts of different massive star
types, and the relative subtype rates of the supernovae which end their lives.
Here we use binary population synthesis models with a binary fraction that
varies with initial mass to test the effects on resolved stellar populations
and supernovae, and ask whether these can constrain the poorly-known binary
fraction in different mass and metallicity regimes. We show that Wolf-Rayet
star subtype ratios are valuable binary diagnostics, but require large samples
to distinguish by models. Uncertainties in which stellar models would be
spectroscopically classified as Wolf-Rayet stars are explored. The ratio of
thermonuclear, stripped envelope and other core-collapse supernovae may prove a
more accessible test and upcoming surveys will be sufficient to constrain both
the high mass and low mass binary fraction in the z < 1 galaxy population.
