Not everyone knows this. I certainly did not know how a fresh fish looks like before I moved to Jelsa. And although I am far from being an expert in this field, I know my basics and so should you, if you want to buy at the fish market.
Here is a great video from Gastro.hr (many thanks to Faros News for sharing), which covers the basics with some helpful visuals. The video is in Croatian, so let me sum up the 3 things to look for:
1. Clear eyes
2. Bright red/pink gills, wet not dry
3. Fish should be firm and shiny
As for octopuses and cuttlefishes, one have to check the eye again – the same as for fish applies, a clear eye is what you are looking for.