The invention is directed to a method for detecting the presence of biofilm-forming microorganisms in a paper or board making process for determining the need of an anti-biofilm agent in the process. The method comprises the steps: (a) subjecting a sampler device in the process line for a period of time to enable said microorganisms to form a biofilm in situ in said process on the surface of the sampler, (b) optionally treating said formed biofilm in a solution of a test anti-biofilm agent for a period of time, then (c) contacting said biofilm with a liquid growth medium in a recession of a culturing device for a period of time, and (d) removing the growth solution from the recession of said device and detecting qualitatively and/or quantitatively the presence or absence of a biofilm adhered on the walls of the recession. An assembly kit is also provided.