Prior to the invention of valve stem seals, oil consumption was much higher, and the lifespan of the valves was far shorter. The seals themselves work on a principle called hydrodynamics. The shape of the seal and the way it performs change, depending on the conditions the seal is under. The pressure and viscosity of the oil vary as the engine operates, causing the performance of the seal to change. As a result, seals often behave differently under different conditions, making them problematic to diagnose.
Start the engine. Check the exhaust for white smoke. If the weather is very cold, or the engine hasn't been started for an extended period, steam from condensation inside the engine can give a false positive. If you see white smoke, gently rev the engine for five minutes, or until the tip of the exhaust is warm. Then try again. Leaking seals cause oil to seep into the cylinder, where it is ignited, causing the smoke from the exhaust.
Drive the vehicle along a straight open road, check your mirrors or have a friend watch behind as you accelerate. Look for more smoke, specifically if it is a darker blue when compared to the white smoke on start up. The increased heat and rpm of the engine while moving will burn more oil at a higher temperature.
Check the oil level. Have you had to add more oil than usual? Is the oil level decreasing at a faster rate than you usually see? If you are not sure, measure your oil level with the dipstick, making a note of the date and time. Use the vehicle normally for one day and check again. Realistically, there should not be a visible difference in oil level over the course of one day, unless oil is leaking.
Start the engine again. Check the idle speed of the engine. Has it risen at all recently? If oil is leaking around the seals, the compression forces in the cylinder are reduced. The engine has slightly less resistance to work against when compressing the cylinder, so it may idle slightly faster than usual.
Examine your exhaust ports. If oil is leaking around the seals and burning, then you will likely find a sticky black residue around the tips of your exhaust. If your vehicle has twin exhausts, check both, as a leaking seal on one side of a V-shaped engine will only foul one exhaust. A dramatic difference in discolouration between the two suggests a problem.