Otter and mink are really quite different - once you know what to look for that is! Otters are native to the UK where as American mink are invasive, having escaped or been misguidedly released from fur farms, and now pose a huge threat to our native wildlife.
What to look out for
Otters are much larger than mink and have a light brown coat, often lighter on the underside. In comparison, mink have glossy dark brown fur that looks almost black with markings on their chin and throat that are bright white and vary between individuals. Mink have short fluffy tails where as otter have large tails with a flat and broad base.
Mink are often described as ferret like, being about the same size as a ferret with a similar pointed muzzle. Otter are far larger. They have broad muzzles, similar to a dog's, and a large flattened head.
As both mammals are considered practically amphibious, you may well spot either in the water! Mink swim with the head and body high out of the water where as otter will swim with only the head and part of the tail visible.