To answer properly, I'll have to tell you about the first time I saw Margaret Howell trousers in a second hand shop about twelve years ago. I stopped to look because I'd heard a lot about her and I thought, 'Is that it?'
I tried them on because I was curious. They were nice, very nice even but I still couldn't see what all the fuss was about. I bought them anyway because they were a good price but I came away telling myself I was turning into a label victim.
Then I started wearing them. And I noticed that they looked as pristine at the end of the day as they did at the beginning; that even if I didn't hang them up, I could grab them the next day, give them a shake, put them back on and that they'd look good. Over time I also noticed that the fabric didn't go bobby or shiny, that they didn't sag around the arse or knees, that buttons stayed in place and hems didn't unravel. I bought a shirt and accidentally washed it on 90 degrees. It was fine.
So no, I'm not confident I can tell just by looking, and I certainly can't tell from a photo or a screen. But with certain things - not all brands by any means - let me wear them for a few days and yes, I think I'd be able to tell.
I also buy a lot of vintage and I can tell if it's genuine 1940s/'50s/'60s pretty much instantly, and certainly as soon as I pick it up. It's not because of the style - because that gets replicated - it's the weight.