Abstract: The application relates to a method for testing a dental instrument comprising the steps of: a) subjecting healthy dentin tissue free from smear layer(s) to treatment with a dental instrument, thereby yielding treated dentin tissue, b) addition of a composition comprising one or more hydrazine derivative(s) to the treated dentin tissue obtained in step a), c) examining if the treated dentin tissue in step b) is stained by the one or more hydrazine derivative(s), and d) determining whether or not the treated dentin tissue contains a smear layer, wherein said one or more hydrazine derivatives is a compound of formula (I) RNHNH2 wherein R is a chemical group containing a chromophore or forming a chromophore with NHNH2. Further, the application relates to a kit of parts for detection and removal of carious dentin tissue and to a method for detection and removal of carious dentin tissue.