Except for gasoline and windshield wiper solution, the fluids in
your car shouldn't get used up or go anywhere. If you notice
that any are low, there's a good possibility of a leak.
Understand that the fluids you may have in your car are gasoline, oil, coolant, brake fluid, windshield washer fluid, gear oil, power steering fluid and automatic transmission fluid. All cars will have at least gas, oil and brake fluid. Air-cooled engines (like old VW bugs) do not have coolant. Your model of car may or may not have power steering or automatic transmission fluid.
Open the hood and visually inspect the engine block and engine compartment. Many leaks are easily detectable with just a simple look.
Note that you don't need to know the name of the fluid that's leaking or the name of the part it's leaking from to be able to find a leak.
Inspect underneath the engine and the car with a flashlight. Look for wet areas or drips clinging to the underside of the vehicle's carriage.
If you don't see any signs of a leak, lay down a large piece of corrugated cardboard and park your car so that the engine sits over it. With a pen, mark the position of the wheels.
Remove the cardboard the following morning. Note the position of any drip marks relative to the wheel markings. This information will help your mechanic diagnose the problem.
