How to Tell if a Car's Water Pump Needs Replacement
The easiest way to tell if a car's water pump needs replaced is to check for leaks. Park your car on a flat surface and place a piece of clean cardboard directly underneath the motor. Let the car sit overnight and then examine the cardboard. If the cardboard is wet, you likely have a leak somewhere in the motor. Try to position the cardboard directly under the water pump, and look for traces of green liquid on the cardboard, which is usually a sign that antifreeze is leaking from the pump.
Let your car sit overnight, parked in a garage with a clean concrete floor. If it's not possible to park it inside on a clean concrete surface, place a piece of light-colored cardboard underneath your car directly under the motor. Be mindful that water pumps are more likely to leak while the vehicle engine is running, so this is a poor reference for finding a leak. Examine the cardboard the next morning. If it appears to be wet from coolant, you have a leak somewhere, possibly in your water pump, but other sources of leaking coolant could be radiator hoses, heater hoses, freeze plugs, gaskets, or the radiator itself. To narrow the search down, try to place the cardboard directly underneath the water pump itself. If you notice green liquid on the cardboard, it's antifreeze. This means you have a coolant leak somewhere. Check the water pump pulley. Find the round part at the front of your water pump that the belt is around. Attempt to rock the pulley back and forth. If it seems to be loose, it may be time to replace it, because the bearing is going bad. Listen to your car. Start your car's engine with the hood up. If you hear a low-pitched grinding noise, it may indicate that your water pump bearing is going bad. You can often hear it plainly if it's gone bad. You also have similar bearings in your AC compressor, power steering pump, and alternator, so you need to be able to focus on the exact source of the sound, often difficult on a running engine. Look for leaks around the water pump. If you see drops of water or a small stream, you have a leak. Many water pumps have a weep hole in front of the seal that will allow water to leak out if the seal is bad. Notice if your temperature warning light comes on. If your car is not circulating enough coolant due to a leaky or malfunctioning water pump, your engine temperature is going to increase, triggering the warning light. Observe if your low coolant light is glowing. This can be an indicator that your coolant reservoir is leaking or that you have a bad water pump. One other alternative is that there is a leak in the cooling system. 
Let your car sit overnight, parked in a garage with a clean concrete floor. Examine the cardboard the next morning. Check the water pump pulley. Listen to your car. Look for leaks around the water pump. Notice if your temperature warning light comes on. Observe if your low coolant light is glowing. 