So, how can you tell? As I said, I'd love to learn to distinguish between species.
Shape (head size relative to body etc.), colouration, spots (location, colour of spots, degree of spotting).
Which species of trout / salmon did you have it narrowed down to?
Look up brown trout (which can look similar to atlantic salmon) to learn how they look.
Brook trout are fairly distinct as far as streams go (in lakes, can be confusion between lakers, splake, brookies).
Coho and chinook salmon, which are plentiful in Ontario, are usually back in the lake before they reach a size that anglers will catch in the streams, or so I've heard.
Not sure what a pink salmon parr looks like but they might also be back in the lake before reaching a substantial size.
Fish identification comes with practice and time. And you can have vastly different colouration and shape within a species depending on each fish, time of year, whether it's a resident or migratory fish, etc. A silver trout or salmon in the lake or ocean will look totally different in full on spawning colours, body shape can also completely change.