Fish is the easiest to assess. The scales will reflect light and be filmed with a slightly slimy sheen; the eyes will be clear and transparent while the gills will be bright red. Above all, it will not smell even slightly fishy. If it does, then it is no longer fresh.
Meat is more difficult, but when buying beef look for a dark red colour with the fat tinted with yellow. If it is too bright then the meat has not been hung for long enough. There is a pernicious practice in some supermarkets of lighting food displays in a way which intensifies the colour, so be on your guard and examine the meat away from the cabinet.