A well-made liner lock is a beautiful thing.  The action is smooth,
the lock is very strong, and it can be opened and closed one-handed.
However, it is easy for the knifemaker to make a mistake on a liner
lock.  Many common mistakes can result in the lock accidently
unlocking, and this is a serious threat to fingers.  Below are some of
the tests we recommend a potential buyer try on a liner lock.  Keep in
mind that many of the factory knives easily pass all the tests below,
while many knives from custom makers -- including those lauded in the
knife rags -- often don't pass.  Test your knives, don't assume the
more expensive knife has the more secure lock-up!

One caveat is that the second of A.T.'s suggestions, the
"palm-on-spine" and "whack-the-spine" tests, are a bit controversial.
We both feel that a blade should never close due to palm pressure, and
a moderate whack on the spine shouldn't make a blade fold up either.
Some makes say that a knife in normal use does not ever get whacked on
the spine, so this test is not real-world.  You can decide for
yourself how secure you think the lock should be.
 

A.T. Barr's tests:

- You don't want your blade to open except when you want it to.
Always check for a good detent ball to blade tang contact.  Open your
liner lock normally and then close it very slowly. The blade *should*
snap closed the last 1/16" or so.

- Open your knife blade very slowly, until the lock engages. Do not
snap it open. You want the tension of the liner lock to just snap to
the tang of the knife. Then do two things. First turn the knife over,
and using the palm of your hand try to close the blade. It should not
close. Then strike the blade spine on the table. Not real hard, but it
needs some pressure. It should not close.

- Snap the blade open REAL FAST, then close it. If it takes a lot of
pressure to unlock the blade, walk away from that knife.

- Open the knife blade real slow, and check for any movement. Sideways
or up & down.

Great tip:
Also, if your liner lock has a sloppy lock-up, sometimes you can help
it by snapping the blade open and then half-way hard striking the
blade (try to close it) on it's tang. That will help seat the Titanium
liner to the tang of the blade.  If that does not work, send it back
to the maker. Be careful when you do this. If the blade does
disengage, the blade will hit your knuckle.  A number of rec.knife
readers have reported good results using this tip.
 

Joe Talmadge's tests:

Open knife, then thumb the lock aside (blade is still open).  Wiggle
the knife back and forth.  If the blade has *any* play at all, that's
a bad sign.  It might just be that the pivot is too loose, so tighten
the pivot until there is no more side-to-side play, and then make sure
the action is still acceptable.  Sometimes a knifemaker will have a
bad action, and then make it appear smoother by loosening the pivot
too much.

On top of that, I do the "white knuckle" test, which many makers also
fail.  Making believe I'm under stress, I grab the knife in a very
firm grip, letting the flesh of my fingers sink in and around the
liner to whatever extent this happens.  Now the question is: will
small movements unlock the lock (if a small movement moves the lock AT
ALL, assume it can unlock it)?  If the lock is too loose or too high
relative to the handle scales, a knife that passes the other tests
might fail this.  I made an expensive folder from a well-known maker
fail this way.  I sent it back to him and he fixed it to my
satisfaction.  That is why I like the AFCK-style handles that do not
give easy access to the lock via a cut-out -- I'd rather it be a
little harder to unlock than to unlock accidently under weird
conditions.