It seems like you're asking about how to test fish, but your question is not very clear.

Look for physical signs of freshness: Fresh fish should have clear and bright eyes, moist and shiny skin, and a mild, ocean-like smell. Avoid fish with cloudy or sunken eyes, dry or discolored skin, or a strong, "fishy" odor.
Gently press the flesh: Fresh fish should spring back when you press it, and the flesh should be firm and not mushy.
Check for gills: The gills should be bright red or pink, not brown or gray.