Beef that's gone off will feel sticky and slimy. It's because bacteria have started to grow and colonise the surface.
Another sign that meat has gone off is the smell. If it has an unpleasant odor it might be off. But remember, meat that's come straight out of a vacuum pack will naturally have a strange odour – until it's been exposed to the air for a couple of hours.
If the meat has green or grey spots, it might have gone bad. And if there's any mould growing on it, discard it immediately.