Make an extract of the bacteria, centrifuge the extract to clarify it, then test various dilutions in an assay for hydrolase acivity using a substrate that increases its absorbance when it is hydrolyzed. For example, 4-nitrophenyl acetate turns yellow when hydrolyzed by an esterase. To demonstrate that it is a serine hydrolase, test for the effect of a serine hydrolase inhibitor.