What is a good way to check if cooking oil in an unopened bottle stored for while might have gone rancid?

Smell Test: Open the bottle and take a whiff of the oil. Has the smell changed or does it have an unpleasant odor? If the oil has a sour, musty, or pungent odor, it's likely gone bad. Any smell other than a neutral, somewhat nutty scent probably indicates rancidity.

Taste Test: Dip a clean spoon into the oil and take a small taste. If the oil tastes bitter or off, it's best to discard it.

Appearance Check: Look at the oil's appearance. Rancid oil may appear cloudy or hazy instead of clear. Sometimes it may also have some sediment or particles floating in it.

Expiration Dates: Check the expiration or "best before" date on the oil bottle. While not an absolute indicator of spoilage, it gives a general idea of the oil's freshness. It's best to discard oil that's past its recommended date.

Remember that these checks work best when you have another, fresh bottle of the same oil for comparison. Also, note that some oils, like coconut oil, may solidify at cooler temperatures. If it looks solid but smells and tastes fine, it's likely still good.