First it's the overall profile and proportions. If that's doesn't grab me in an instant then I don't give it any further consideration.
If it does, then is the knife aesthetically pleasing?
Then I go to the overall design. Is it interesting? Is it timeless? Is it well thought out? Is the design "fit for function" or is it designed to perform to a high degree? Is the design ergonomically pleasing?
Is the knife made of desired and quality materials?
Is the overall construction, fit and finish executed to a high degree?
Does the maker of the knife strive to advance and perfect his skills? Does he promote himself and the custom knife community? Does he continuously work to make a market for his and his collectors knives?
If I were to have to sell the knife at a future date would I at least recoup the money I spent or preferably make a profit?
It takes me about 10 minutes to insure a knife meets these or the majority of these criterior and if it does I "Putt the Trigger"!