Sometimes I can tell. Usually you get better detailing with the expensive stuff, in the same way that you get nicer interiors on a Lexus than a Toyota.

Granted the difference in quality is definitely not worth the difference in cost (like many luxury goods you'll end up paying 5x in price for what feels like a 1.2x improvement in quality).

I recently replaced 20 or so $10 target shirts with a handful of $60 shirts from Nordstrom (NM sells them too) and they're much more comfortable, hold up better in the wash too.

Absolutely not, that's why I'm questioning why any of these are being described here as "luxury brands." A t-shirt from a luxury brand could cost several hundred dollars, none of these stores carry anything like that. This is all shopping mall grade.

The grass is always greener...

Someone out there is wearing haute couture hand-sewn by artisans. Your hundred dollar shirt is laughable to that tier. But for the majority of the world, I think Nordstrom would largely count as "luxury"