Look for good heat treatment and good quality steel. Steer clear of makers who are ambiguous about steel type or heat treatment. Something made from "the finest German stainless steel" is made from mystery steel. Something made from "S30V" will have properties that you can look up or ask folk about. Steer very clear of "secret formulas." Legitimate cutlers have nothing to hide and often take delight in debumking "secret formulas."

Look for good fit and finish. Steer clear of gaps or voids between parts of the knife (that eliminates my stuff, for now at least). Also be wary of such voids or gaps having been filled with epoxy or other sealer. Good fit and finish is a sign of attention to detail. Attention to detail is required from a quality maker.

Contact the maker, whether a factory or an individual and ask them questions about their product. If they make odd claims ("our swords ahve been famous for 2600 years" or "no other maker does this") then leave them be. If they simply give you facts and offer more help or advice, then seriously consider their product.

One hears a lot about how flexible a blade is, which results from a misunderstanding of the ABSA's (American Bladesmith Association) bend test for awarding journeyman or master status. A good blade does not need to bend 90 degrees without breaking. In fact, unless a knife has been designed for a function where a lot of flexibility is required (say a fish skinning knife ???), blades should be fairly stiff. A lot of flexibility is typically associated with either a poor choice of materials or over temperring.

The ABSA's bend test, is a test to determine one's skill at differential heat treating (hard edge/soft back). Done correctly, the soft back supports the hard edge thereby preventing it from breaking. The best example of this are well made katana - their edges are often in the 60-65 Rockwell C range but they have soft backs which gives flexibilty and strenght (part of that is due to engineering beyond differential heat treating). Knives that are Rockwell 60+ throughout often break easily - they have great edges but break.

Quality knives can be had from many, many of todays makers. Also most are available to speak with if what you are looking for is a custom piece vs a catalog piece.
I have a personal relationship with all of the makers who's knives I own and carry.
Just off the top of my head:
Mick Strider and Duane Dwyer Strider Knives
Ernest Emerson
James Coogler CnC Knives
Mark Terrell
Warren Thomas
Shane Seibert
Lucus Burnley
Eddie Baca

There are many knife forum/ discussion groups out there. Out of respect I won't link them here, but an internet search will reveal much.

Also there are many reputable purveyors(sp?) that not only will be happy to assist you in your purchase, but also have many makers whose knives they stock.

Again off the top of my head here are some very honorable dealers that I do buy from:

Triple Aught Design Gear www.tadgear.com
Blade Art www.bladeart.com
PMDC Tactical www.pmdctactical.com
EDC Knives www.edcknives.com