To your first question, a look is suggestive to me if it's a look that you wouldn't be sharing with someone that you're married to.  I mean, just the word "suggestive" means that you're suggesting something– and in today's terms it means that you're suggesting something intimate.

Photography is tricky like this, for the whole part of modeling is putting on a face that say something– that attracts attention– and that means that photographers try to get models to make facial expressions that evoke desire in men.  It's part of the "selling sex" that a lot of marketing does today.

As far as whether a guy can tell if a girl isn't intentionally being seductive, I'm not sure that this has ever been field tested– I'm sure there are some girls that are good enough actresses, but I was never good enough to fake like I liked someone, even when I was trying.