Answer:
The strength of a tornado is determined in the aftermath by examining the damage done by the storm. This is used to provide a wind speed estimate and give the tornado a rating on the Enhance Fujita scale from EF0 at the weakest to EF5 at the strongest.
Tornadoes are detected through a combination of doppler radar and weather spotters. Radar can detect the wind velocity signature of a tornado's vortex, but visual confirmation is needed to confirm that a tornado has touched down as radar usually cannot scan the lowest portions of a storm.
Tornadoes on the long term by analyzing certain weather conditions such as temperature, dew point, wind shear, and whether or not any storm systems are present. This cannot tell where a specific tornado might hit, but it does say what regions might see tornadoes in the following hours or days. On the short term it is possible to scan a thunderstorm with doppler radar and determine whether or not it might produce a tornado.