How can we tell sea trout and brown trout apart?

It is sometimes quite difficult. Fresh-run, silvery, black-spotted, sea trout are obvious, but maturing specimens gradually darken on their return to fresh water, becoming more and more like brown trout.  This fact, coupled with the extensive morphological variation among brown trout, can make recognition tricky.  Normally, adult sea trout are bigger than brown trout found in the same waters, but there are many exceptions.  Male sea trout tend to lack the truly red spots found on many resident brown trout, although they often have orange ones.  Other useful indicators of brown trout, such as haloes around spots and black and white edging to fins, are not always present.  Scale reading by experts provides some scientific validation or, more certainly, biochemical analyses for levels of strontium in scales (or bones), or marine oils in tissue samples.  The species composition of internal parasites is another useful indicator of feeding at sea, but this is another test that requires the specimen fish to be sacrificed.  In practice, judgement by anglers as to whether a coloured trout is a sea trout or a brown trout must take account of the likely prevalence of sea trout in that system.