Simply put the dimensions of the solar panel you are looking at into the form below, and compare against the advertised wattage.
If you have a major variance, then there is as good chance the panel you are looking at is not the rated wattage.
PLEASE NOTE : Solid panels will actually read lower on this calc than solar blankets etc, this is because this calculator doesnt take into account that solid panels are generally more efficient. A new calc with better options and accuracy will come soon!