How can I find out which lab delivers the highest quality porcelain veneer?

The short answer is that you can have the best lab in the world, but without the best dentist, you may have a mediocre veneers.  The way the doctor cuts the preps, determines whether your veneers will be bulky, misaligned, wrong shade, mismatched, etc.  This has everything to do with the doctor an little to do with the lab.  However, having a competent lab is important.  I suggest shopping around online for the doctors with the best before and after photos of patients.  If that doctor does fantastic work, then they are obviously using a great lab as well.I hope this helps,Dr. Sarah Thompson

Let your cosmetic dentist be your guide. If you like the work that they have done, they will explain the different labs that they use. 
I commonly use three labs, as veneers and the aesthetic challenges can be graded. Not everyone has the same needs and demands, so although you need a Rolls Royce level of workmanship, offering a slightly more affordable 'high-level' is still excellent for many people. 
Basically for labs, time-taken adds cost and level of detail, naturalness and beauty. 

Good luck in finding your treatment provider. 
Kind regards
Thomas Dey, BDS, MSc
  

Hi:
It would be very difficult for you to find out which lab is the best.  The questions you can ask about the lab is, if your work is being done by a master technician and look at the quality of his work to see if you like it.
Best,
Dr. Maddahi