I have knives worth thousands of dollars, but mostly from a collector's stand point, as they aren't much different from a sixty dollar knife.
I have lots of high end stuff, but mostly carry an 84mm SAK, a 40 dollar Kizer, or an old electrician knife. Losing my a sprint run Spyderco kind of broke me on expensive knives with pocket clips.
What you are looking for is dependent on knife type:
Traditional slipjoint or lockback:
pull weight/walk and talk - not too strong, not too light. Half stops?
blade centering, blade rub on multiblade slipjoints
flushness of backsprings - are they proud when open? Can you see light through the liners?
fit and finish - scales mated to bolsters, matched on each side, pins flush, peened properly. Blade kick proper so blade isn't proud open but doesn't wrap backspring
Quality of material. Sambar stag, bone, exotic wood, filework backsprings, shield shape, stamp cleanliness, bolster embellishments, etc.
Modern folding knives:
Blade steel is one of the biggest ones. Lots of guys care a lot about this shit, but all they cut are flat rate boxes with their new knives they just ordered inside. Nerding out on steel for the sake of doing it is fun, but it isn't practical and is honestly horseshit for 90% of the knife carrying population
Blade centering.
Smoothness of open, close, and lock mechanisms
Fit and finish, similar to above
Exotic materials.
If you're ever in Lincoln Co, hit me up and I'll let you see some knives. I have modern Chinese folders that far surpass USA made midtechs and customs. The knife market is amazing right now with high quality Chinese knives at a decent cost (Kizer, Civivi, etc), nice Italians (Lionsteel, Maserin, Fox), traditional French and German knives, etc.