Women constantly check each other out. Like
@AJ84 said it’s interesting when you see an attractive couple to wonder about the dynamic.
Attractive people garner attention on their own. Women check out other women in couples for a couple of reasons.
1. To compare themselves to the chick a man is with physically
2. To assess if they are better/prettier/thinner than said chick
3. Out of envy or jealousy
4. Out of curiosity
5. Out of attraction/admiration of the woman in question
Those five things can skew either positive if the woman making the observation has good self esteem (e.g. She has great legs...I wonder what does she do to keep in shape)...or can skew negative if the woman making the observation is insecure or low self-image (e.g. What does he see in her...she’s *insert XYZ insult*)
The insecure woman will mentally tear the subject chick down to make herself feel better, especially if the observer woman sees the man as desirable and his woman/date in a less favorable light than her false self.
And sometimes you’ll have women who are overtly complimentary. They will say things like “It’s obvious why he’s in love with you, blah blah blah.” Sometimes those women are genuinely giving a compliment...other times they say it whilst holding a symbolic dagger to backstab you because they have an agenda.
It really depends on the woman. But women are always aware of other women around them and also aware of relationship dynamics around them.
We come equipped with stock issue radar for that. (Shrugs shoulders)