Do you mean D-amino acid aminotransferase or D-amino acid transaminase? Check the literature on the enzyme for assay methods. For D-amino acid aminotransferase, some of the literature references can be found in this database entry.

https://brenda-enzymes.org/enzyme.php?ecno=2.6.1.21#SYNONYM

Adam B Shapiro Thank you for your response, I mean D-Amino Acid Aminotransferase.

The reaction is D-alanine + 2-oxoglutarate pyruvate + D-glutamate

How you measure the reaction depends on what equipment you have available. My preference would be to measure the reaction continuously by following the production of pyruvate with a lactic dehydrogenase-coupled enzyme system. Lactic dehydrogenase and NADH are commercially available. The necessary equipment is a spectrophotometer or absorbance plate reader.

pyruvate + NADH lactate + NAD+

This reaction is observed by the decrease in the absorbance of NADH at 340 nm.