Can ultrasound determine if a mass in stomach is a tumor or bezoar?

16 year old male neutered cat, has been healthy until a few months ago, when he started vomiting and showed decreased appetite. Have taken him to the vet 4 times. Xray showed a mass in stomach. Today they did an ultrasound and basically said it’s a mass in stomach.. suspected stomach cancer. A year ago the cat had xrays that were perfectly normal (unrelated issue that quickly resolved itself).
The vet has done no bloodwork (he is a conservative vet & said the bloodwork from last year was normal).
He said we could opt for a $2k exploratory surgery- does this mean the ultrasound today was meaningless? He said he suspects the large mass could be inoperable due to the possibility of it being near a bile duct valve.
My question is, could this mass be a bezoar? What other tests could be done to determine if this mass is a tumor, and if the ultrasound can’t tell the difference between a cancer & a bezoar what was the point of the ultrasound?
Thank you