A liner lock has a scale on the lock side.
A frame lock doesn't so you can see both the short and long cut of the lock and usually the detent ball hole if it was drilled clear through.
I will say that the idea that a frame lock is strong, or more specifically stronger than a liner lock is not entirely accurate. In my own testing of knives sent to me for evals by manufacturers some liner locks held more free weight on the tail end of the folder than so called 'heavy use' frame locks. This translated directly to the lock cuts made to spring the locks to the proper tension to make contact with the blade when opened. In my experience some liner locks of .070" thickness with no lock cuts made to spring the locks (IE, Buck Strider folders like the 881 for example) were stronger than some .125 frame locks with .032" lock cuts made to spring the locks. Strength is not necessarily the frame locks forte` when taken to the extreme thinness some are taken down to here on the back part of the lock but that is not really all that needs considered and fact of the matter is they are still plenty strong enough in most all uses, and yes even some heavy ones and even when this thin but perhaps reliability is more of a factor going for the frame lock but as Artfully Martial addressed even that can be debated some.
In frame locks and thicker model liner locks the lock cuts again translate to how easy the lock moves side to side to disengage the lock to close the blade. In my own experiments with lock thickness at the cut to spring the lock the thicker you make them the more tension that lock will have. Some left real thick can actually off center the blades when closed on some models and can be terribly hard on the thumb to release at all, let alone by accident. The best ones seem to find a happy middle ground that works best. Sebenzas are there IMO> Of course other factors come into play with how easy or how much resistance a lock has to move side to side laterally. Length of the long cut, width of the lock, lock cut thickness to spring the lock, how much spring and whether the lock sticks or not to the contact on the blade all play into this as well.
Thicker lock cuts like those seen on say, the Sebenza models, the Camillus Cuda Max models, or even A.G. Russells SeaMaster folder are harder to move sideways to disengage the lock but not so bad they off center the blades or are hard to negotiate. The clips are made such as to allow ample hand and finger contact to the locks during use except A.G's which doesn't come with a pocket clip.
What A.G. Russell's SeaMaster does sport is a lock block to prevent hyper extension of the lock out the wrong way. This can be one of the drawbacks to a frame lock with no scale there to prevent over travel of the lock which can at worst eliminate the proper spring tension altogether and sometimes weaken it to where you notice vertical blade play after hyper extending it. I've had some mailed to me to be resprung for this from guys using frame locks without the pocket clips which happened to be positioned such as to allow the clip to act as a secondary stop for blocking lock over travel. Remove them on the ones with super thin lock cuts and you end up with that easier lock manipulation backfiring on you. This can take place from an adrenaline rush in a heated moment or just as easily from a very sticky lock that needs more force to free up the blade to close it. Those thin lock cuts do make the larger frame locks work more like their liner lock cousins but its a double edged sword so to speak.
Personally I have never had anyone report to me or read hear of frame locks disengaging in use with the hand gripped on the folder contrary to numerous reports of liner locks defeating but I do suppose that if the knife twisted in your hand during use and your finger slid over the lock as the knife moved the correct way in your hand it is very possible that a frame lock or liner lock, particularly one with the lock release raised above the scales, where the bottom part of the lock is exposed, that the lock could very easily be moved toward the release motion and possibly even disengage freeing up the blade. With that said, personally I'd sooner hard stab any frame lock than a liner lock. I'd also add, how often have your knives twisted or shifted in your hands during use causing the lock to move any direction except toward the stop pin? It could be argued that even with the knives like some of Terzuola's liner locks with the lock side exposed some which opens up the lock on both the non lock and lock side could in some eyes add reliability. Does it make it more possible to disengage the lock? I would say only in the rare occurrence that the whole body of the knife shifted or twisted in the hand and even then in the correct direction to disengage the lock. If it twisted the other way it could just as easily engage the lock further in securing it better. It is also highly likely that the fingers could just as easily keep the lock from moving at all.
Lock defeats would be where a choil would come in handy and its one of the reasons I like the Strider SnG and PT and put them in my top five of best designed frame locks. ( not to mean to miss the others offering choils) Even if the lock did disengage during use, if you had your index firmly wrapped around that choil ahead of the pivot like the design allows it would still be very hard for the blade to close on your fingers or hand and bite you. That is something that even the Sebenza cannot provide. Liner locks with choils are my favorite type liner locks also. Second would be liner and frame locks with flippers because these flippers, although fun to use to open the blades also add a notch of safety that is flying below most folks radar. The flipper actually is somewhat of a block for blade travel too. Take your JYDII by Kershaw, Tyrade frame lock or Zero Tolerance 0301 or 0302 and you can immediately see why these are also tops in my book of well made highly recommended frame and liner locks.
Lastly it seems to me that frame locks do seem to wear a bit better than thinner liner lock models particularly seeming to hold up better to sudden shocks that can indent the thinner ones but both, when done correctly are quite nice and certainly worth having in anyone's collection.
Anyway, thats my .02 cents worth on it.
STR