1. Hot Springs and Geysers: These are the most obvious signs. Hot springs occur when heated groundwater rises to the surface, while geysers are hot springs that erupt periodically, driven by steam pressure.

2. Fumaroles: Fumaroles are vents in the Earth's surface that release steam and gases, often containing sulfur dioxide, hydrogen sulfide, and carbon dioxide. The presence of these gases is a clear indicator of geothermal activity.

3. Altered Vegetation: Geothermal activity can cause changes in the vegetation, such as stunted growth, unusual coloration, or even the presence of heat-loving plants that are not normally found in the area. This is due to the altered soil chemistry and temperatures caused by the geothermal activity.