Hexoses are mainly found in nature in the pyranose form (6-membered ring). Yet, furanose forms (5-membered ring) are observed in some rare polysaccharides. Using IRMPD spectroscopy (InfraRed Multiple Photon Dissociation), we propose a straightforward diagnostic of the ring-size of N-acetyl galactosamine ions. The furanose form of N-acetyl galactosamine was synthesized and its protonated ion was isolated in an ion trap to measure its gas phase vibrational spectrum by IRMPD. Comparison with the IRMPD spectrum of its pyranose counterpart reveals that they have distinctive optical fingerprints. This new MS-based diagnostic opens the way to facile identification of the ring-size in oligosaccharides. Our experimental data also provide new insights to support the theoretical description of the conformational behavior of the furanose ring, which is notoriously more flexible than the pyranose form but remains difficult to assess.