Three tests:

1. Put some cal hypo powder on a stained area. (Cal hypo is usually available as bagged shock. You can probably buy a single bag at a pool store.)

2. Put a few Vitamin C tablets (ascorbic acid) on a stained area.

3. Put a few tablespoons of Iron Out (sodium hydrosulfite + sodium metabisulfite) on a stained area.

See what the results are.