when the girl or the woman is wearing a men's watch, that should not be a problem... I remember two times asking about a women's watch: once was in a hospital where I had a general short check by a woman doctor (age about 30) wearing a rolex submariner. Of course I had seen which brand/model it was, nonetheless I asked her if her watch were the Rolex Submariner, which she confirmed and showed it to me. So we had a very short but interesting small talk about the submariner before the next patient was coming...
The other time I remember was at the butcher's shop, where the sales girl (around 25 I guess) was wearing a men's size Festina Chronograph which I didn't recognize at first sight. Anyway, after having bought what I wanted I asked her something in the style like "I'm sorry, but would you mind telling me what kind of watch you are wearing? It is looking quite cool". She looked happily and told me about the brand and furthermore that she liked it because it was bigger than watches specially for woman, which I obviously found cool as well...
I guess this way it is quite easy to approach a woman. I would even say - if you are single and looking

- that asking about the watch could be an ice breaker - compared to all those standard approaches... - to have a first contact to that woman, but surely not to overstress her by telling you of your fetish at this point..., nor to reduce that woman to the watch... Sure it is nice if the woman is congenial AND wearing a nice watch
Regards, wl69