How Do You Tell If Synthetic Oil Is Going Bad?

This is the guideline I go by, just from my own personal experience.
If you check your dipstick and the oil is either a golden or brownish color, then it has very little contamination and is therefore still good. If the oil has turned black but you can still see the markings on the dipstick through the oil, it's still safe but you should consider changing it soon. If the oil is overdue for a change, then it will be completely black and you won't be able to see anything on the dipstick itself. If it has been really badly neglected, then (when the engine is cold) you may notice more carbon residue towards the bottom of the dipstick, indicating that the detergent additives in the oil have been depleted and are no longer capable of holding the contaminants in suspension.

Is no way to tell if motor oil of any kind is going bad by just looking at it. There are expensive tests which will tell you just about anything you want to know. Just prior to the car sitting for the six months changing your oil so the engine has clean oil in the crankcase is about the best favor you can do for your car. In the past have had many vehicles which I stored over our long winters and made the clean oil deal my number one priority.