my music instructor tells me that my bass guitar is going flat?

[FirstNameprajeetM]

i own an ibanez gsr200 (made in china) my music instructor is telling me that my bass is going flat he's comparing it to a yamaha rbx170 which he has. how do i determine whether my bass is in fact going flat?? frankly i feel the yamaha sounds very jazzy.........................fyi my instructor has about 5yrs of experience in the field well actually hes a guitarist but plays bass too...............and btw when i went to buy my bass a semi-pro helped me buy it and i dont think he was affiliated with the store in any way.