Yes it often is but I've had dates and SO's ask me that question with it not turning it into a trap. It might just be the women I date but it some women (straight) do check out other women and are curious to hear guys opinion about such.
I was asking if your SO has asked the question not you asking your SO. Although I could see how that could work into a strategy by asking SO about what she thought of what a woman was wearing, her hair, her purse or some other accessory. That way you could distract your SO into a commentary on fashion while you scope out the figure inside the fashion.
just go avoid giving an answer and say something insulting about her. It can be anything, so long as it true. It could also be insulting to how she is dressed.
"She was okay.... but I couldn't get past the webbed feet." or... "Honestly, I didn't notice what she looked like. When she walked by, that perfume / body odor was just overwhelming. I'm feeling light-headed."
of course...women are quite capable of spotting beauty in other women. there are some she wouldn't even ask me about...she'd know i find the girl attractive...for others, she might ask because it's "close." :grin: