DC Motor as a Generator Voltage and Current
Yes, if the reverse EMF of the motor and its DC resistance are specified.  Good datasheets do provide these figures.

At first approximation, a generator looks like a voltage source proportional to speed in series with the DC resistance of the windings.  Put another way, you can usefully model a motor as a Thevenin source.  Good datasheets tell you the voltage as a function of speed, or "reverse EMF" as a function of speed for a motor (same thing).  That's the Thevenin voltage.  The resistance, again from the datasheet, is the Thevenin resistance.  You can then compute voltage and current as a function of electrical load from there.

Sometimes these parameters can be inferred from other specs.  For example, consider a motor that puts out 100 W of mechanical power at 60 Hz rotation (3600 RPM) speed, at 24 V applied, and is 90% efficient.  The electrical input power is (100 W)/90% = 111 W.  That means the current is (111 W)/(24 V) = 4.6 A.  The winding resistance is then (11 W)/(4.6 A)&sup2; = 518 m&Omega;, which is dropping (11 W)/(4.6 A) = 2.4 V.  That means the back EMF at 60 Hz is 21.6 V, or 360 mV/Hz.

If you were to drive this motor mechanically at 40 Hz, it would look like a 14.4 V source with 518 m&Omega; in series.  If you were to put a 1 &Omega; load on it while still keeping the  rotation rate at 40 Hz, you'd get 9.5 V and 90 W.