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)² = 518 mΩ, 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 mΩ in series. If you were to put a 1 Ω load on it while still keeping the rotation rate at 40 Hz, you'd get 9.5 V and 90 W.