1. Hot Springs and Geysers: These are the most obvious signs. Hot springs occur when heated groundwater rises to the surface, while geysers are a type of hot spring that erupts periodically, shooting hot water and steam into the air.

2. Fumaroles: These are vents in the Earth's surface that release steam and gases, often with a strong sulfurous odor. They indicate that the underground temperatures are very high.

3. Solfatara: Similar to fumaroles, but these vents emit gases that are rich in sulfur dioxide, hydrogen sulfide, and carbon dioxide. They are often found near volcanic areas.