Yeah, I mean if I open a capsule, I can tell. But without opening it, is there any way to tell it's rancid?
The pure oil is cheaper and you can feel its taste without having to break capsules. Moreover you do not have to ingest capsules made of dubious substances, besides the desired oil.
Once opened (but preferably also before) such an oil bottle should be kept in a refrigerator.
Some bottled oil includes flavors, e.g. lemon, which mask its natural taste. Unflavored omega 3 oil is preferable.