How do I know if the lab my dentist uses for veneer fabrication is a quality lab?

While it really comes down to trusting your dentist, you COULD ask what lab is being used and google/contact them.  Not all labs are equal.  There MAY be a surcharge to pick a lab if you don't like their chosen lab.

Not all porcelain veneers are created equally, and ultimately the success of any porcelain veneer case depends on 3 factors 
1 ) Communication - You and your dentist need to be on the same page before any treatment is started. Its important that your desires and goals are understood, otherwise you may not get the results you're looking for. 
2 ) Skills of the dentist - Since most porcelain veneers are permanent restorations, its important to seek out an experienced cosmetic dentist who has successfully treated many cases that are similar to yours. Generally a dentist who limits their practice to cosmetic dentistry usually has completed many hours of additional training. These Dr's should be trained in using the best materials and and work with quality labs that will deliver beautiful long lasting results.
3) The Ceramist - The nicest porcelain veneers are hand made by a lab ceramist who works with dental porcelain. The ceramist is an artist who sculpts and layers porcelain in order to recreate the natural beauty of enamel. Depending on their skill level, your veneers can look so natural, that no one would know you had them placed. I can't emphasize enough how important the ceramist is in the process.
Its best to have a conversation with your dentist and ask him what lab he uses for porcelain veneers. Many top ceramist and labs can be researched online and have reviews posted. Also, look at the dentists portfolio of work. Most people can differentiate an excellent lab from an average one by looking at their work. Lastly, read the dentists' reviews and see what their patients are saying about the work and their experience.

Hi Love2829
The most important thing to have is trust in your provider. When it is someone you know and have a relationship with... then the hope is that you are in safe hands.
But perhaps you are a bit new to the area and/or practice. A good way to start the conversation may be "Dr. X... how do you choose which lab you will use for making veneers for me?"
I think the answer will be very telling. Hopefully the doctor will have had a long and great relationship with the lab they work with. And if they do give you options for different tiers of labs... ask which lab they would prefer if they were having the veneers done on themselves.