How to detect if meat (or any product) is not safe to be prepared as food?
Visual signs and smells are an unreliable source for making this decision.  Foods can be contaminated, and/or unsafe, and you will be unable to tell by looking at a product or smelling it.  The only reliable ways to be safe are to (a) understand the source of your food, (b) ensure (or assume) that it has been prepared and stored properly before it got to you, (c) pay attention to packing dates and shelf life, (d) store the food properly yourself, and (d) cook your food properly.

According to foodsafety.gov (USA), in general, raw ground meat, poultry, and seafood has a refrigeration shelf life of 1-2 days. Raw roasts, steaks, and chops (beef, veal, lamb, and pork) have a 3 to 5 day shelf life, and cooked meat, poultry, and seafood can be stored in the refrigerator 3 to 4 days.