Back on the topic of finding out a partner's weight:
My ex refused to tell me her weight. When I looked back at older pictures, I could tell she had gained a significant amount of weight, but I can only gues. I only found out my ex'es weight when she had moved out and I found a WLS referral letter that she had forgotten to take with her.
Approaching the topic can be tricky, especially when you have not been together yet for very long.
With my current wife, I was affraid she might get self concious about it, so I never really approached the subject. I only found out when she wanted to go on a diet for our wedding. She did a starting weigh-in and measurements, and let me help with those, so that was kind of fun (even though the numbers were shrinking in the coming months). She has been more open about her weight since then. Pregnancy was also fun, because she was doing belly measurements to track progress.
Any way, my advice is to approach the topic with caution and to just be patient.
Alternatives could be to go to a recycling station with her where the car is weighed before and after, and then afterwards return without her "because you forgot something".
Other idea: our local zoo has a kind of scale where you can step on that will then say that you weigh "0.05 elephants" or something like that. Maybe she is willing to step on something like that togther with you