Methods for identifying the presence of cancerous cells or tissue by: applying a compound of formula (I), or salts thereof, to a living body, wherein R1 represents hydrogen or the same or different one to four substituents; R2, R3, R4, R5, R6, and R7 independently represent hydrogen, hydroxy, alkyl, or halogen; R8 and R9 independently represent hydrogen or alkyl; and X represents C1-C3 alkylene; and detecting fluorescence emitted by a compound of formula (II), wherein R1, R2, R3, R4, R5, R6, R7, R8, and R9 are as defined above; wherein fluorescence emitted by a compound of formula (II) is indicative of the presence of cancerous cells or tissue.