When brazing HVAC linesets, the correct method is to flow an inert gas through the line in order to prevent interior oxidation of the copper and brazing products. However, as a specifier, I can say that this needs to be done until I'm blue in the face, but contractors (in my experience) are notorious for skipping this step. It adds cost, time, and a system where this is not done will not fail immediately, but it will contribute to the premature failure of the system. In the long term, this simply looks like a system that ran it's course, rather than the product of poor installation practices. Is there a way to tell by inspection after installation that this practice was followed, or are you at the mercy of the contractor if someone is not watching?
You could do a visual inspection with a pipe camera on the interior. Poorly-brazed copper will have discolorations / flaking due to oxidation (just like the exterior would). You can also just pull one of their brazes and slice it in half to inspect it. This PDF ("Brazing Best Practices: 12 Tips for HVAC Technicians" from Nov. 2014 issue of Welding Journal) has a good photo of what you'd see from a bad braze: https://app.aws.org/bsmc/WJ_201411_62.pdf
The pipe camera obviously is only cost-effective if you've got a lot of subcontractors doing brazing work that you need to check (or if you already have one). Slicing is probably easiest. You could also supply them with the nitrogen and check the tank pressure afterwards - obviously they could just bleed it for a little bit, but at some point fooling you is going to be more of a hassle than just doing the job correctly.