Did the bulge or disk of the galaxy form first? How is this determined?
The bulge, by comparing oxygen:iron content of stars in the disk and the bulge.
In star formation, "Oygen is predominantly produced in the explosion of massive, short-lived stars (so-called Type II supernovae), while iron instead originates mostly in Type Ia supernovae, which can take much longer to develop."
Research has shown that stars in the bulge are more oxygen-rich compared to stars in the disk. This means the stars in the bulge are older, thus meaning the bulge formed first. Interestingly, this also proposes the idea that the bulge formed independently of the disk.
All information can be found in the link above.
I hope that helped!