SloopJohnB@mac.com

Inspect the boots every time you wash the wheels! If you see lines of grease, etc., on the wheels you have a leaking CVJ boot.
The inner boots are a little tougher to inspect...you have to get under the care to look at them...generally every time you have the car up on a rack or you're under there to change oil. If you change oil from the top with a suction pump, stop it! Get under there at least once a year and/or every oil change to inspect the inner front boots.
IF you see a tear or cracking at the bellows you should remove the bad boot and install a new one. The outer ones can generally be replaced without removing the axles BUT it's difficult to clean the outer CVJ without removing the entire axle. The CVJ should be cleaned before installing a new boot unless you catch the damaged/cracked boot before the lube gets contaminated so badly that you can't simply wipe off good lube and reinstall a new boot and lube. If the lube feels sticky/gritty you'll have to clean the CVJ.

SloopJohnB@mac.com

Most commercial mechanics prefer to replace the entire axle...they get a profit from the axle since they can buy it generally at a 40% discount plus there's a heckuva lot less cleanup/mess involved...they just pull the axle and replace it with the rebuilt axle. The old axle generally has to be pulled anyway so why not just put in a rebuilt and avoid cleaning/lubing etc. a new one. The exception is when only the outer CVJ needs to be replaced....if the splines are undamaged the old outer CVJ is simply popped off the axle (using the axle bolt and a short socket inserted into the hole the axle bolt is screwed into to pop off the outer CVJ or a gear puller on some axles without axle bolts) and a new CVJ installed.