Vehicle engines include a dipstick that allows you to determine the amount of oil present by withdrawing a slender rod and observing how much of it is slick with oil. Compare the level of oil on the rod with the maximum oil capacity of the vehicle.
When oil capacity information is not available in the vehicle's user manual, determine maximum and minimum oil capacity through the Internet. By consulting with the manufacturer's website or with reliable third-party websites to determine how many quarts of oil for which particular engine is designed, you can compare the general level shown by the dipstick reading. For example, a 4-quart engine with a dipstick reading showing oil capacity at roughly 50 percent shows that the vehicle has about 2 quarts of oil.
There is no standard location for a vehicle's dipstick; different cars, trucks and motorcycles all feature these tools in widely varying locations. HowStuffWorks suggests consulting with the owner's manual of the vehicle in question to determine where the dipstick is located. The owner's manual often offers more information about oil capacity and preferred ratings as well. The site mentions that a slight excess of oil is not dangerous, but clearly states that low oil levels require an immediate refill.