Wine is made through a fermentation procedure of the sugars in grapes developing into alcohol. In time, that fermentation procedure will continue and the wine will turn into vinegar. That's why an old, unopened bottle of wine will likely be vinegar.
see post You can inform if wine has spoiled if it has a distinct vinegar odor, if it tastes undesirable, or if it has actually altered color. It's also a sign that it's gone bad if it has any mold growing on it.
If you're uncertain if your red wine has gone bad, it's better to err on the side of care and discard it. Red wine that has actually spoiled will not make you ill, but it's not going to taste good either.
To avoid your wine from going bad, make sure to keep it appropriately. Red wine must likewise be stored upright so that the cork doesn't dry out and let air into the bottle.