Being able to identify the peak for carbonyl groups on infrared spectrographs is one the handful of characteristic peaks that the MCAT does seem to expect students to have familiarity with. Carbonyl groups give a strong absorption at 1700 cm-1. For aldehydes, specifically, in 1H NMR spectroscopy, because of the strong deshielding brought about by the electron-withdrawing carbonyl oxygen, the aldehyde proton is shifted far downfield (d = 9 to 10 ppm).
Infrared spectroraph of vanillin.




Infrared spectroraph of vanillin.