Just chew a few fish oil capsules and you can see if they're rancid or not. Mine have always been fine.

BTW: turns out my various joint/muscle/tendon issues were due to a couple of things: Gluten intolerance and ankylosing spondylitis. Unfortunately a lot of the low carb products have a lot of gluten in them. And the thing that makes AS worse is starches. Any time I eat starch my aches get worse.