A method of identifying a liquid composition comprising a tracer compound and at least one original liquid ingredient which is present in the liquid composition for a purpose related to the intended use of the liquid composition, the at least one original liquid ingredient being a liquid or a dissolved form of a compound which would be a solid but for its inclusion in the liquid composition, said method comprising the steps of: a. adding the tracer compound to a known liquid composition containing said at least one original liquid ingredient to form a tagged known liquid composition; b. obtaining a sample of an unidentified liquid composition; c. optionally, diluting said sample of an unidentified liquid composition by mixing with a liquid; d. contacting said, optionally diluted, sample of the unidentified liquid with a spectroscopy-enhancing surface; e. obtaining at least a portion of a Raman spectrum from the sample of the unidentified liquid composition including at least a first peak attributable to the tracer compound and a second, different, peak attributable to the original liquid ingredient; f. calculating, from said spectrum, the concentration of the tracer relative to the concentration of said original liquid ingredient in the sample of the unidentified composition by calculating a peak height or peak area ratio from the first and second peaks of the unidentified liquid composition; g. obtaining at least a portion of a Raman spectrum from the tagged known liquid composition including at least a first peak attributable to the tracer compound and a second, different, peak attributable to the original liquid ingredient; h. calculating a peak height or peak area ratio from the first and second peaks of the tagged known liquid composition; and i. comparing the relative concentration calculated in f with the corresponding relative concentration of the tracer and the original liquid ingredient of the tagged known liquid composition. a. adding the tracer compound to a known liquid composition containing said at least one original liquid ingredient to form a tagged known liquid composition; b. obtaining a sample of an unidentified liquid composition; c. optionally, diluting said sample of an unidentified liquid composition by mixing with a liquid; d. contacting said, optionally diluted, sample of the unidentified liquid with a spectroscopy-enhancing surface; e. obtaining at least a portion of a Raman spectrum from the sample of the unidentified liquid composition including at least a first peak attributable to the tracer compound and a second, different, peak attributable to the original liquid ingredient; f. calculating, from said spectrum, the concentration of the tracer relative to the concentration of said original liquid ingredient in the sample of the unidentified composition by calculating a peak height or peak area ratio from the first and second peaks of the unidentified liquid composition; g. obtaining at least a portion of a Raman spectrum from the tagged known liquid composition including at least a first peak attributable to the tracer compound and a second, different, peak attributable to the original liquid ingredient; h. calculating a peak height or peak area ratio from the first and second peaks of the tagged known liquid composition; and i. comparing the relative concentration calculated in f with the corresponding relative concentration of the tracer and the original liquid ingredient of the tagged known liquid composition.