I am working on a project in which the performance of three types of
fuels will be compared by testing the performance of each fuel in
small RC two-cycle engines.  I will be testing RC glow fuel in an
unmodified glow engine in addition to diesel and biodiesel fuel blends
in a modified RC diesel engine.
I would like to know how to characterize the fuels which we will be
testing.  Specifically, I am interested in the relevant physical
properties of each fuel that could be used to predict how that fuel
would perform in an engine.  I am attempting to establish correlations
between performance parameters (i.e. power output, RPM, fuel economy,
as well as emissions) and the fuels? physical properties (i.e.
viscosity, iodine number, cetane and octane numbers, etc.).  Could you
provide me with your expertise as to which of the physical properties
would best correlate to the engines? performances?  If the last
paragraph or so is not answerable, is there an ASTM method for
determining the quality of diesel fuel?

ASTM D975 is your standard diesel specification other relevant ASTM tests are:

Cloud Point - D2500, or D5773, D5772, D5771
Pour Point - D97 or D5949
Cold Filter Plugging Point - D6371
Low Temperature Flow Test - D4539
Distillation - D86 or D2887
Density - D1298 or D4052
Flash Point - D56 or D93
Viscosity @ 40°C - D445
Sulfur ppm wt. - D5453
Sediment and Water - D2709
Ramsbottom carbon residue on 10 % distillation residue - D524
Ash % wt. - D482
Copper Strip Corrosion - D130
Cetane Number - D613
Conductivity - D2624
Lubricity @ 60°C - D6079