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The correct term for this is dynamo (or generator), although it's the same most of the time. It depends on how much water will pass, what the pressure will be etc. what kind of dynamo you've to use. Perhaps you can use the dynamo of an old bicycle?
When the dynamo spins, an AC voltage will appear on the output. By rectifying and smoothing it you can get a smooth DC voltage (the same happens in almost every electronic device connected to AC power) which you can measure with a multimeter, microcontroller, IC, etc.
When you already have a motor, you can test if it works as a dynamo as well by connecting the + and - to a multimeter (has to be set to AC voltage). Make the motor spin very fast (really fast for some motors!) and check if there is an AC voltage. If so, it works as a dynamo. If not, it doesn't (or you have to spin faster)