Otters certainly do leave scales and general fish debris where they devour their catch, and, as long as you keep your distance, would have no concerns about entering your pond.
If you know where the animal enters your property, lay down a sand trap which it has to walk over, this will allow you to check its prints.
Also the Otter is much larger than the Mink with a longer tail and white throat.
If you come across any droppings (spraint) they are black, pencil thin, and usually in a prominent spot, smell them!! Otter spraint smells lovely, really, all sweet and fishy, but Mink is a harsh ammonia aroma.
Mink are wanton killers, and as such have many enemies, but you can't blame an animal for doing what comes naturally, where they enter an Otters territory they come off second best and are seen off!!
Good luck with your spotting, the two animals are not really alike, I'm sure you'll know as soon as you see something.
Keep us informed please.....All the best
