Connect 24V and measure the no-load speed with a tachometer - that gives you
a good estimate of the motor constant. Measure the resistance and you'll get
a reasonable idea of power level (assume 10 to 15% of power wasted in windings
for a small motor at a reasonable operating point).
The motor constant can be measured either in Nm/A or V/(rad/s).
You can find the no-load current trivially.
Efficiency for a small motor is somewhere about 60 or 70%, but is less for
a gear motor, depending on the number of gear stages (each stage might be 70%
in itself)