In the present invention, for test cells which are either stem cells whose state of differentiation is unknown or cells obtained from stem cells by differentiation induction, an LC-MS or GC-MS analysis is performed on culture supernatants collected from a culture dish of the test cells and a culture dish of control cells whose state of differentiation is known, and the state of differentiation of the test cells is assessed based on the amount, determined as a result of the aforementioned analysis, of at least one compound selected from the group of putrescine, kynurenine, cystathionine, ascorbic acid, riboflavin, pyruvic acid, serine, cysteine, threonic acid, citric acid, and orotic acid in both the culture supernatant of the test cells and the culture supernatant of the control cells.