Shock absorbers are basically "dampers." The springs on a vehicle's suspension absorb bumps from the road by compressing, and the shock absorbers are responsible for the amount of bouncing created when the springs rebound.
There are several warning signs that can tell you when shock absorbers need replacing, such as your car bottoms out over speed bumps or dips in the road or keeps bouncing way more than it usually is supposed to. However, other signs, such as excessive body lean in turns, unusual noises while going over bumps or that the front end of the car dives sharply in hard braking. The key factor here is that, since shocks wear out gradually, you might get used to a bouncier ride, a lot like getting used to longer stopping distances as brake pads wear out.
One quick way to test this is to push down hard on each corner of a vehicle; if the vehicle continues to bounce after you let go, then you need new shocks. Nonetheless, this test requires quite a bit of strength, and with many high-riding vehicles, it isn't easy to get the leverage you need.
The wise thing to do is to have a qualified mechanic check your shock absorbers, preferably when your car is hoisted up on a lift. Shock absorbers are filled with fluid, so, the mechanic will be able to see if there are major leaks, worn mounts or bushings.
Despite the manufacturers advice to replace shocks at specific intervals, when you need to do it can vary by vehicle and how and where you drive. If your daily route takes you over rough, pockmarked roads such the Dhaka streets, then that put more stress on the shock absorbers. In this case you will need to replace the shocks more often than you would have had to if you drove on smoother roads. Also, keep in mind that hauling heavy loads will also wear out your shocks faster.