Open the Whirlpool microwave's door and look on the bottom front frame. There will be a model number plate attached here.
Look for either "Output," "Watts" or "Wattage" on the label. The number listed next to this is the wattage; it can be anywhere from 300 watts to over 1200 watts.
Look in the owner's manual for the Whirlpool microwave if there is no wattage designation on the microwave itself. The manual will list the model's specifications, including the wattage.
Look for the model number on the model number plate if you don't have the manual. Call Whirlpool at 866-698-2538 and tell them the model number. A representative will be able to tell you the wattage of the Whirlpool microwave.