When sourcing perfume oils, the primary quality indicator is the concentration level, typically categorized as Grade A (Premium), Grade B, or Standard. You should prioritize oils that are 100% undiluted (Pure Oil) and free from alcohol or water fillers. Request a Gas Chromatography-Mass Spectrometry (GC-MS) report to verify the chemical composition and ensure the scent profile remains consistent across different batches. Additionally, check for longevity (skin retention time), which for high-quality oils should typically range from 8 to 24 hours depending on the base notes.