Measurement of motor wattage
You have to extrapolate from some measurements.  The physical size will give you some idea of how much power the motor can dissipate itself without frying.  Look at specs of other motors that are about the same size and overall construction.

You say it's "burning" on 14 V.  So don't do that.  Use a variable supply and slowly crank up the voltage until you think the motor is running as fast as it should.  That may be hard to guess, but it should not be dissipating significant power then.  If it gets really hot at 14 V, then don't give it more than 12 V.  Maybe it's a 9 V or 6 V motor.  Does it seem to run "fast" unloaded at 9 V or 6 V?  When slowly ramping up from 0, at what voltage does it start spinning with no mechanical load?

Once you think you have the operating voltage, find the current draw at that voltage with the shaft held fixed.  That is the stall current.  That times the voltage is the worst case power the motor will dissipate at that voltage.  Also measure the unloaded current at the same voltage, and the spin speed under those conditions if you can.  That will give you some idea of the efficiency and the internal generator constant.

Give it the maximum voltage you determined and hold it there with the shaft frozen for a little while.  Keep monitoring the temperature.  If it seems to be getting hot fast, turn it off.  If it seems to be able to take it long enough to reach equilibrium temperature, then you've found everything you need to know.

If the motor gets too hot, then either your top rated voltage guess was too high, or this motor isn't intended to be run at stall indefinitely.  Use as appropriate to what you find.