If you meant meteorologist a meteorologist is a specialist who studies processes in the earth's atmosphere that cause weather conditions.
The most important tool in monitoring tornadoes is Doppler radar, which can detect tornadoes and the rotation that can cause them. They also use modern communication systems, which allows people to quickly relay eyewitness reports.
Meteorologists are not always 100% right because the weather forecast that is given is based on an estimation of weather patterns in the state and region the forecast is for. Weather is also extremely unpredictable so it is very difficult to get it accurate all the time.