Look at it. Smell it. If it passes those national-security-level tests, heat a bit, cut up a slice of bread and fry a couple of croutons. If they look and smell and taste good, we'd say the oil is OK. If they smell or taste rancid or seem headed in that direction, we would invest in new oil.

Old oil or oil that has been used before is less hearty than fresh oil. It is likely to start smoking at a lower temperature than fresh oil, and, pushed too far, will start to burn, will burn the food you're frying, and eventually could burst into flames. You should always be watchful and monitor the temperature and cooking process when frying foods, but you must pay even closer attention when using older oil.