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Motor Voltage

I have a few small motors that have no specs available. I can measure the internal resistance and calculate the stall current for a given voltage or actually measure the current draw under stall conditions using say 5 volt supply to motor. But how can I determine the ideal voltage range to drive a motor without any specs?

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It all comes down to how much heat the motor windings can dissipate. If it's not too unrealistic, try to measure the gauge of the winding wires - you can look up online to find the recommended max current.

Another method is to connect the motors up to a variable power source, and gradually increase the voltage. Bring the supply up by a volt or so, let the motor run for a minute, and then check the temperature. Eventually the motor will have trouble removing enough heat from the windings, so the idea is to stop raising the voltage when the casing becomes quite warm.
Naturally this approach isn't very precise, but it's better than nothing =)