Most laboratories have access to a UV-visible spectrophotometer. A simple assay for plasmin uses a synthetic peptide substrate that produces an increase in absorbance when cleaved by plasmin (see link below). This is the simplest quantitative assay method I can think of. If you don't have access to a spectrophotometer, you can still assess activity qualitatively by eye by the increase in the yellow color due to peptide cleavage.

http://www.sigmaaldrich.com/technical-documents/protocols/biology/enzymatic-assay-of-plasmin-with-d-val-leu-lys-p-nitoanilide-dihydrochloride.html

Thanks for the reply! I did not know about this assay. We have a plate reader spectrophotometer as well as fluorometer. We had been using a kit, but it is so expensive to order kits over and over.

I agree