The rattlesnakes I have crossed slithers with start rattling when they get concerned I was too close to them and continued for a while. When that happens I stop dead and use the rattle to locate it, which I have usually been able to fairly quickly, but they blend in so well to their surroundings it can take a few minutes. I will then back away and if I can move around it safely I will, otherwise I wait for it to move off on its own.
Where I tend to keep a sharp eye out is at the edge of the trail where there is some light huckleberry or brush overhanging rocks the sun is hitting. They will sometimes coil up there to warm up and can be very difficult to spot if they don't rattle (some won't).