To practice and hone this ability, Padawans of the Jedi Order had to relax their perceptions so that everything surrounding them becomes a blur. With a lot of patience and practice, the Force would reveal itself and let the practitioner see an area's Force signature and revealing whether it is sickly, beneficial or malevolent.
In some places where the dark side of the Force is quite strong, this sense provides the Force-sensitive with a bit of a warning before approaching the site. Sometimes, beings could sense the alignment of other Force users, like Yoda did to Dooku. Those skilled in the Force could also try to counteract this power by hiding themselves, or being near a nexus of the side they were opposed to.
During his Jedi training on the swamp-planet Dagobah, Luke Skywalker used this Sense-based Force power when approaching the Dark Side Cave on Dagobah. He later commented on it in the The Jedi Path: A Manual for Students of the Force.
Being guided by the voice of Qui-Gon Jinn's spirit, Master Yoda also came across the Dark Side Cave where he had a vision of the future Great Jedi Purge and an illusion of Darth Sidious telling him to join him.
Qui-Gon Jinn has been practicing this ability on Coruscant and appeared amazed by the amount of life at the planet.