Assume two circumstances, one easier than the other.
A) She is pretty
B) She is not pretty
How do you answer this question from a spouse or S/O?
Context matters most. If she's talking about someone on TV or is showing you a picture of someone she knows, I'll answer yes. Past experience with other girls has me avoid adding any degree to it...you can start getting yourself in trouble by calling her "hot" for example.
Random girl on the street that you 100% noticed and she's trying to see if you paid a little too much attention to her...I'd tread more carefully.
I'd answer honestly on the negative too except for friends of hers...I'll just say not my type.