If there's excessive wear to the clutch, an increase in revs won't be matched with a corresponding increase in speed.
Try putting the car into a high gear at a low speed. e.g. put it into 4th gear at 20mph. Then put your foot down. If the car struggles up to the correct speed, the clutch is fine. If , however much you increase the revs, the speed doesn't increase, the clutch is slipping and won't last much longer.
When a clutch finally packs up, you start to notice a drop in speed as you're driving along. e.g. If you were driving at 70mph, you find that your speed has dropped to 60mph. You put your foot a bit further down and your speed goes back to 70mph. A little further along, you notice another drop in speed and, once again, you press the accelerator pedal further down. Eventually, even with the pedal on the floor, you can't keep the speed up and it gradually drops lower and lower. At about this time, the car starts to fill with a dry, acrid, burning smell. At this point, get off the carriageway as soon as possible because you're definitely not going much further!
Chris