I was told if you tie a overhand knot in it and pull if it holds its good if not throw it away.
Basically you cannot tell how bad a line is until you put a stress in it like tying a knot and checking the tension break at the knot.
Most lines are rated at or just above the rated pound strength. When you tie a knot or put in kind of bend in the line then the line will more than likely fail at or near the point of line change. A knot drops the line pound rating by 10-20% at the knot or "twist" of the line into the knot.
For instance if you fish with 20 lb test the line will probably hold 20-24 lbs. But if you scrape or damage the line or a fish nicks it it will break at the abrasion point. We all know that. When you put a knot in the line the knot now goes from 20-24 lbs to 16-20 lbs.
2 strke is correct. tie a knot and pull hard to see if it breaks. Test with new line to get a feel for how much it takes to break.