If you notice that your truck is low on coolant, there are a few things you can do to check the level. First, check the overflow tank. If the coolant level is below the full line, then your truck may be low on coolant. Second, check the radiator cap. If the radiator cap is not seated properly or if there is coolant leaking from it, this could also indicate that your truck is low on coolant. Finally, start the engine and let it idle for a few minutes. If you notice that the temperature gauge rises into the red zone or if steam starts coming from under the hood, this means that your truck definitely needs more coolant.