You might notice that your engine just doesn't have the power it used to. You might even notice that you have a surging or skipping in higher gears. You also might notice that the RPMs go way up when you shift into a new gear. These are signs that your clutch might be going out. A good test is to find an empty area in a parking lot somewhere. With the engine running and in a higher gear, like second or third, release the clutch with the brake applied. If the engine stalls, your clutch is likely good. If the engine keeps running then you might need a clutch. A worn clutch can also cause a distinctive smell. It's difficult to describe other than it smells burned. If you find your clutch is worn, it should be replaced. Here's a video on this procedure.