How do you know if meat has gone bad?

Look for a shiny almost petrol like colour on the meat, look for dis-colouration or a funny smell

Right after the animal is slaughtered that when it’s bad. Bad for you, bad for the environment, bad for everyone. Grow up and stop exploiting animals and the planet because you just feel so so special and believe everything revolves around you and that the universe owes you anything. 

Usually you will have abdominal pain and cramps followed with diarea

It stinks/smells rotten.  That's how you know

it usually doesnt erect

Meat that has gone bad might smell rotten if you taste it and it has a funny taste or a fizzy taste you discard it and you wash your mouth out really good.  Unfortunately you can expose meat to salmonella bacteria or other bacteria by leaving it uncovered while it's defrosting. And then decide oh let me put it in the refrigerator and I'll use it tomorrow.

This happens all the time.  The meat doesn't smell bad it doesn't taste bad but when you eat it you have a god-awful case of food poisoning.

You can tell, by the way it looks and smells.

It gets slimy and bacteria bad spoiled smell. 

If it's brown before cooking and smells funny

The flies will love it.