John: Since you're in-process of your build I'd remove any accessible trim panels up front and look for leaks yourself. Once those are spotted or located they're somewhat easy to repair without enlisting a W/S technician, a DIY operation. Attempting to seal a leaking W/S or the surrounding "frame" (pinchweld in glassman's jargon
) from the outside is at best difficult, more often than not pretty much worthless.
If you guys need or want the best method to determine how to find the source of your leaks I can write something up. It can be a one person job but a second hand never hurts.
BoyWonder: It is 50/50 whether the upper body seams can be fully inspected by raising the windshield molding. The actual glass of the W/S pretty much fills the opening and as such the seam could be separated where its not visible from outside. The only real sure test shy of pulling the W/S, looking around and making repairs as needed is the water test.
On another note: While its possible to successfully remove an existing W/S without it breaking its very much a long shot; no honest glass shop will guarantee this. W/S's are prone to aging which affects their strength, the pressures applied as part of the removal process can break them, the older it is the more likely this is to happen.
Also most glass shop technician's are in a big rush to start and finish a job so you being present and asking them to pause for most any reason isn't part of their daily schedule. IF a body leak is present and after several attempts to DIY reseal it don't work its time to at least consider a new W/S. In this situation you'd negotiate to have them remove the windshield and install a new one after you've dealt with the body leaks. Most shops might have a small charge for this as a two operation job but if that can be rolled into the complete price it would be fair to all.
As for quality I'd avoid at all costs the brand Safelite jams on customers, their "SGC" brand. Truth is that stuff is complete crap to the point their better installers almost always refuse to use it. Its cheap for Safelite however if you're attracted to their "lifetime, nationwide warranty" go for it-----its not the worst policy for those not in the glass biz.