How to tell if the FWD constant velocity joint needs replacing?

Noise is usually the most obvious clue. Check the following list of symptoms:

• The classic symptom of a worn or damaged outer
  joint is a popping or clicking noise when turning. The symptom can be
  aggravated by putting the car in reverse and backing in a circle. If the
  noise gets louder, the outer joint’s should be replaced.
• A "clunk" when accelerating, decelerating or
  when putting the transaxle into drive. This kind of noise can come from
  excessive play in the inner joint on FWD applications, either inner or outer
  joints in a RWD independent suspension, or from the driveshaft CV joints or
  U-joint in a RWD or 4WD powertrain. The same kind of noise can also be
  produced by excessive backlash in the differential gears.
• A humming or growling noise, sometimes due to
  inadequate lubrication in either the inner or outer CV joint, is more often
  due to worn or damaged wheel bearings, a bad intermediate shaft bearing on
  equal length halfshaft transaxles, or worn shaft bearings within the
  transmission.
• A shudder or vibration when accelerating may be
  caused by excessive play in either inboard or outboard joints, but more
  likely the inboard plunge joint. These kinds of vibrations can also be
  caused by a bad intermediate shaft bearing on transaxles with equal length
  halfshafts. On FWD vehicles with transverse-mounted engines, this kind of
  vibration can be caused by loose or deteriorated engine/transaxle mounts. Be
  sure to inspect rubber bushings in the upper torque strap on these engines
  to rule out this possibility.
• A vibration that increases with speed is rarely
  due to a bad CV joint or FWD halfshaft imbalance. A missing damper weight on
  a halfshaft can sometimes cause harmonic vibrations, however. An
  out-of-balance tire or wheel, an out-of-round tire or wheel, or a bent rim
  are more likely causes.
• If a joint seems noisy, a visual inspection
  should follow. If the boot is loose, split, cracked, torn, or punctured,
  chances are the joint is noisy because it has lost its supply of grease
  and/or the joint has been contaminated by dirt and/or water. Either way, the
  boot and joint will probably have to be replaced. If a joint isn’t making
  noise but the boot is damaged, the boot should be replaced immediately. If
  the grease feels gritty, dirt has gotten inside the joint. Chances are the
  joint has already been damaged. Either way, the joint should be
  disassembled, cleaned and inspected before the new joint is installed. CV
  joints require a special high temperature grease. Ordinary chassis grease
  will not do.
• CV joint repairs should not be put off. Failures can have serious results. An outer
  joint that seizes while driving can cause loss of steering control. A joint
  that fails and breaks apart may cause the driveshaft to drop out of the car.