Water can be used in place of engine coolant, but it is not recommended. Radiator coolant is designed to keep the radiator contents from freezing and the engine block from cracking. It is also raises the boiling point of water to help keep the engine from running too hot.
Check coolant levels by looking at the overflow tank. There should be visible markings indicating whether the coolant is at an appropriate level. Some older vehicles do not have visible overflow tanks. When this is the case, open the radiator cap and look inside to ensure that the amount of coolant it contains is sufficient.