The paper presents a novel approach to quantifying molecular complexity using assembly theory, which estimates the complexity of a molecule by finding the shortest path to construct it from building blocks. The authors demonstrate that the molecular assembly index (MA) can be experimentally measured using three techniques: nuclear magnetic resonance (NMR), tandem mass spectrometry (MS/MS), and infrared spectroscopy (IR). By analyzing the number of absorbances in IR spectra, carbon resonances in NMR, or molecular fragments in MS, the MA of an unknown molecule can be reliably estimated. This method provides a first experimentally quantifiable approach to determining molecular assembly, which could be used to explore the evolution of complex molecules and identify evolutionary processes. The study includes theoretical and experimental validation, showing strong correlations between the inferred MA and the actual complexity of molecules. The combination of multiple spectroscopic techniques further improves the accuracy of MA inference, making this approach valuable for exploring chemical space and detecting biosignatures in environmental samples.The paper presents a novel approach to quantifying molecular complexity using assembly theory, which estimates the complexity of a molecule by finding the shortest path to construct it from building blocks. The authors demonstrate that the molecular assembly index (MA) can be experimentally measured using three techniques: nuclear magnetic resonance (NMR), tandem mass spectrometry (MS/MS), and infrared spectroscopy (IR). By analyzing the number of absorbances in IR spectra, carbon resonances in NMR, or molecular fragments in MS, the MA of an unknown molecule can be reliably estimated. This method provides a first experimentally quantifiable approach to determining molecular assembly, which could be used to explore the evolution of complex molecules and identify evolutionary processes. The study includes theoretical and experimental validation, showing strong correlations between the inferred MA and the actual complexity of molecules. The combination of multiple spectroscopic techniques further improves the accuracy of MA inference, making this approach valuable for exploring chemical space and detecting biosignatures in environmental samples.