In reply to higherclimbingwales:
Shoulder straps often have a shorter padded bit, be narrower and be shaped (all to avoid boobs). There is sometimes more of an S curve to the back to accommodate the fact women have larger bums
But more extensive differences in some brands than others. At the cheaper end of the market, I suspect they're often the same apart from colour.....
Try on and see if it fits. I know at least one bloke who uses a Lowe Alpine girly pack like mine because it was cheap in Keswick and didn't realise it was a female specific one (being an alpine one, rather than a rambling one, they made it in grey.....)