Check the inside of the oven's door, on the serial number plate on the back of the oven, or in the owner's manual or the manufacturer's website for the wattage.
To estimate wattage you can do a "Time-to-Boil Test:"
Measure 1 cup water in a 2-cup glass measure. Add ice cubes; stir until water is ice cold. Discard ice cubes and pour out any water more than 1 cup. Set microwave on high 4 minutes. Watch the water through the window to see when it boils. If water boils:
in less than 2 minutes = very high wattage oven (1000 watts or more).
in 2½ minutes = high wattage oven (800 watts or more).
in 3 minutes = average wattage oven (650 - 700 watts or more).
in 3 - 4 minutes = slow oven (300 to 500 watts).
If your microwave's wattage is lower than the wattage mentioned on the food package cooking instructions, it will take longer than the instructions say to cook the food to a safe internal temperature. Always use a food thermometer to ensure a safe internal temperature.
For more information on using a microwave oven, go to Cook It Safe