If we can't calculate the power using those variables means how can we choose the right motor? The volt meter n ammeter can be used after buying only..So wat can we do if the chosen motor is under powered or over powered for our model after buying the motor?

We go by the published information about the motor.  Start out by the AUW of your plane, find a motor with the appropriate wattage/thrust and go with it.  Overpowered is not a problem , you can always fly at lower throttle.  Overweight is a problem. 

If you feel the setup is underpowered, you can try reducing the AUW (a smaller battery etc).  You can also play around with prop choices.

That has been my grouse. Many motors do not have ANY published information!

The wattage/thrust and recommended prop is available most of the time.  Information like what is the amps draw for various prop choices and lipo cell counts are not always available.

Actually it is a good idea for LHSes to make such information readily available on their websites, as that helps assure the buyers.