You can try to commonly available products to test for metals: Vitamin C (ascorbic acid) and Iron-Out (sodium sulfite + sodium hydrosulfite). If neither one lightens the stains, it probably is NOT a metal stain.

Unfortunately some non-metallic stains can be very chlorine resistant. But, using a sequestrant, chelant, or reducing agent (sodium hydrosulfite -- dissolves metal salts or stains) will have NO effect on a non-metallic stain.