To re-use oil safely, use these tips:
Strain it through a few layers of cheesecloth to catch any food particles. Be careful with hot oil, though, because you can easily get burned.
Shake off excess batter from food before frying it.
Use a good thermometer to fry foods at 190°C.
Turn off the heat after you are done cooking. Exposing oil to prolonged heat accelerates rancidity.
Don't mix different types of oil.
Store oil in a cool, dark place.
Avoid iron or copper pots or pans for frying oil that is to be reused. These metals also accelerate rancidity.
Signs of Deteriorated Oil:
Oil darkens with use because the oil and food molecules burn when subjected to high/prolonged heat.
The more you use an oil, the more slowly it will pour. Its viscosity changes because of changes to the oil's molecular structure.
Loose absorbent particles accumulate as sediment at the bottom of the storage container or are suspended in the oil.
When smoke appears on the oils' surface before the temperature reaches 190 degrees C (375 degrees F), your oil will no longer deep-fry effectively.
If the oil has a rancid or "off" smell or if it smells like the foods you've cooked in it, it should be discarded.