Problem: Extinct or Dormant?
It is difficult to distinguish an extinct volcano from a dormant volcano, since geologic time must be considered. Thus it can be considered extinct than likely a volcano only when the last eruption dates back hundreds of thousands of years. For example, the volcanic system of the island of Gran Canaria has been active since about 15 million years, interrupted by up to 4 million years long rest periods. So it is quite possible that seemingly extinct volcanoes erupt again.
In order to arrive at a reasonable estimate of the likelihood of future outbreaks, it requires a detailed analysis of volcanic activity over the lifetime of the volcanic system. The most important parameters are the distances between the past eruptions (length of rest breaks ).
The fact that they are extinct volcanoes in the volcanoes of the Eifel area in Rhineland -Palatinate, is not scientifically proven. The eruption of the Laacher See volcano is back only 12,900 years and a new outbreak is quite possible.
Physically, is a volcano off when no more eruption is possible by the lack of magma supply.