How to know if the generator is dissipating or generating power?
Desperado, you actually asked when a voltage/current source is a source and when - a load. You are right about the voltage source - it becomes a load when the current enters (instead to exit) the positive terminal. This will happen if we connect to it (through a resistor) another voltage source with higher opposite voltage ("+" to "+", and "-" to "-").

It seems, as you noted, that in the case of the current source it is the same... but I think it is not... Usually, a current source consists of a voltage source and a resistor in series.  So it will act as a current source (will pass a current in the same direction) even when we connect a voltage source with opposite polarity... if only it is lower than its internal (excitating) voltage.

From another viewpoint, a current source (and a real voltage source with some internal resistance) always dissipates power in its internal resistor...