This paper may explain the intersubject variability: Dietary restriction impacts health and lifespan of genetically diverse mice
They found that:
Responses to DR vary across individuals, and the mechanisms underlying this variability remain largely unclear. Studies in mice and non-human primates have shown that the effects of DR are influenced by individual characteristics including sex, body size and composition, and genetics
There is a lack of knowledge regarding physiological markers that predict how individuals will respond to DR. Identification of such predictors could help to tailor DR to individual needs, serve as tools in the longitudinal evaluation of intervention success and elucidate the biological processes that mediate the effects of DR on lifespan.
As GLP-1RAs largely mimic calorie restriction, the same reasoning should apply. In the future a blood test might determine whether someone would benefit from them. In the meantime, I’d rely on “individual feeling” as suggested by @mccoy.