Provided are a method for accurately evaluating the differentiation status of stem cells by selectively staining only stem cells in an undifferentiated state, and a method for positively isolating only stem cells in an undifferentiated state. Specifically provided is a method for determining differentiation of a cell comprising a step of contacting a test cell with a probe comprising protein (A) or (B) below and a step of detecting the presence of binding of the probe to the test cell. The method for determining differentiation of a cell is capable of detecting the presence or absence of an undifferentiated stem cell in test cells by using a probe that specifically reacts with undifferentiated stem cells and detecting the presence of bonding to the test cell. (A) A protein comprising an amino acid sequence shown in SEQ ID NO: 1 and recognizing a sugar chain structure of "Fuc±1-2Gal²1-3GlcNAc" and/or "Fuc±1-2Gal²1-3GalNAc;" and (B) a protein comprising an amino acid sequence showing 80% or more similarity to the amino acid sequence shown in SEQ ID NO: 1 and recognizing a sugar chain structure of "Fuc±1-2Gal²1-3GlcNAc" and/or "Fuc±1-2Gal²1-3GalNAc."