Scan her for a microchip - often spay status is recorded on there if they are chipped at the time of spay or after.
Request any paperwork from prior vet treatment from the person you've adopted her from or at least the clinics name/number so you can ring and verify what if anything has been done.
Check for an ear tattoo - some clinics (depending on where you live, it is standard practice in Aus and was optional in the US when I lived there, may be standard now) place a small tattoo in the inside of the ear which indicates spay status.
Failing that rather than an x-ray, use an ultrasound. Most vets will have access to these now and while they're not often the lastest and most high tech models compared to what's available for human diagnostic use, they will certainly be able to determine if she's spayed or not.