Usually by its minimum surface pressure (at the center of the the storm). The lower the central pressure, the stronger the storm. This includes hurricanes/typhoons and large winter-spring type storms with long fronts attached. The National Hurricane Center will use the Saffir-Simpson scale to report to the public because it's easier to understand. But within each scale, there is a range of pressures/wind for the same scale category, but internally they actually pressure as the measure. Sometimes a tornado is called a cyclone. The hardcore science community doesn't do this but sometimes news reporters call a tornado a cyclone. If you're wondering about a tornado, its strength is measured by observational evidence of its power. For instance, if pavement is torn from the ground it's assigned an F5 (the highest rating) on the Enhanced Fujita Scale and corresponds to winds near 300 mph.