How do you know whne your shocks need replacing?

i recommend after 75000 miles. check for leaks around shock. rubbing, which causes bare metal to show. rust around shocks. do a bounce test: push a corner of vehicle down and then let up, does the vehicle oscillate(bounce) up and down more than once the vehicle is let go. if yes, change shocks. if the shocks are good, the vehicle will bounce once and level out.