It is harder to tell on pickups... I generally can tell by the bounce when hooked to a trailer. otherwise, I go by the bounce test, instead of by mileage.
my ranger still has the front factory shocks, 19 years and 146,000 miles, doenst bounce more than once. the wife's '97 S-blazer has 190,000 on it and doesnt bounce more than once, factory original shocks also. i would have changed tons of shocks over the years if i went strictly by milage. granted the blazer "feels" like a set of shocks would be a good idea. but it passes the bounce test and the tires aint wearing funny.