Can using the motor as a generator be used as a way
to determine the approximate proper voltage and current?

That will tell you rpms per volt. Stall current against a dyno will give
you torque per amp. There's your characteristics.

Now, you have no way to tell what maximums are appropriate, as these have
nothing to do with what the shaft does versus what the wires are doing.
However, the shaft size roughly indicates the max torque, compare what you
have to similar motors in catalogs to get a rough spec. Max voltage has to
do with the insulation and other build features, max rpms and max power
likewise with heat dissipation and the like, you can't measure these things
directly.

Many DC motors are 90 VDC or 180 VDC for a max speed of 1800 or 3600 rpm,
so if you know or guess max voltage or speed you can determine the other.