Failover Routing Question

I usually use SLA monitoring to do failover routing if a link is down. I have a site where the equipment there does not have the ability to do this and the budget does not allow for adding additional licensing for the SLA monitoring at this time.

I want to use weighted static routes to achieve this. My question is how does the routing device (cisco 3850 L3 switch) determine the route is no longer working and use the route with a higher administrative distance? Does the interface have to be marked as down?

Also how does it know to fail back to the original route?

I tried googling this but without a satisfying result.