How are rovers used to detect life on Mars?

The Mars exploration Rovers do not have the ability to detect life directly, yet they are able to provide information about the planet's history. Life, as we understand it, requires water, so the history of water on Mars is critical when trying to determine Mars could ever support life. The task of the rovers will be to collect surface samples and reaveal the mineralogy to see if water was involved. The rovers will be used to determine the location and time frame at which water may have been present.