I love to use the deep fryer to make hot wings. A deep fryer has a basket and every few times you use it, you must take out the basket, drain the old oil, and clean the deep fryer, refill it with new oil and put the basket back in. The oil will smell differently when it goes bad. To me oil that has gone bad smells like paint thinner. If the oil doesn't smell appetizing it is probably gone bad. You have to make sure the basket is not too full or the food will not cook properly. When I put wings into the basket, I dump them in, close the fryer, and hit another button to release the basket down into the oil. I can tell its working right because the deep fryer will sizzle real loud. You may want to time how long it takes once you put the food in until it gets done. Then you can set a timer for that time for each additional batch. Also the sizzle of the food will change as it gets done. With practice you will be able to determine when the food is done by the sizzle.
Deep frying is a cooking method whereby food is submerged in hot oil or fat. Because of the high temperature involved and the high heat conduction of oil, the cooking process is extremely fast. Because no water is used, deep frying is best classified as a dry cooking method.