Checking coolant level is a simple job, but it has one hard rule: only do this when the engine is completely cold. A hot cooling system is pressurized, and opening it while hot can spray scalding coolant.
With the engine cold, locate the translucent plastic coolant reservoir (not the radiator cap itself) — it has "MIN" and "MAX" markings on the side. If the level is at or below MIN, it needs topping up.
Use the coolant type specified in your owner's manual (or a universal long-life coolant), pre-mixed 50/50 with water, or diluted concentrate to the correct ratio — never top up with plain water alone as a permanent fix, since it dilutes the corrosion protection and freeze/boil protection.
Add slowly until the level reaches the MAX mark, then replace the cap firmly. If you find yourself needing to top up frequently, that's a sign of a leak somewhere in the system — hoses, radiator, water pump, or a failing reservoir — and it's worth having a pressure test done rather than just topping up indefinitely.