I'm not sure about the difference between a person who dresses slutty and a person who dresses sexy, but I think it might be whether or not she dresses with class or not. A sexy dresser might wear a short, skin-tight but pretty dress, a slutty dresser might be in booty shorts and a leopard print spaghetti strap top with half her thong showing. Something like that.
As for the time of year thing, it is 105 degrees on average this week where I live. I will wear as little as possible and nobody has the right to tell me that I'm dressed too slutty.
Dressing sexy is dressing in a way that accentuates a woman's features all while keeping it tasteful and classy... Dressing slutty is simply dressing to accentuate a woman's features while lacking in good taste/class.
The way a woman carries herself can has an influence on her overall impression as well... Body language is definitely apart of a woman's look/outer-appearance.
This is how I judge it for myself. I make sure to only show off one asset at a time.
Ex: I do have one of those see through lace shirts, it hard to keep it unslutty but I think I make it work by wearing it with a long flory skirt or high waisted jeans- so the only thing shown off are boobs. If I wear a short skirt, I wear a higher neck shirt. If I'm wearing a really tight dress the neck isn't really low and the hem isn't really short.
Slutty is showing everything, sexy is hinting at everything you got by giving a taste