The most simple and faster method is using a "selective" culture media. Prepare an agar plate with 1% Cellulose Avicel or Carboxymethyl cellulose, and then inoculate with the sample of soil previously dissolved in water (can you perform a dilution and plating technique from the soil samples). If the fungus can degrade cellulose will produce cellulases, because the cellulose is the only carbon source.  This method remains 3 or 4 days ( time to grow of the fungus)

The culture media only should include Agar (2%) and Cellulose and Carboxymethylcellulose (1%)

Do you have the fungus already isolated?

Dear colleague, thank you for your answer. It seems to be a very fast method. I isolated some strains, including genera like Aspergillus, Penicillium, Cladosporium or Fusarium. And we want to selected strains which have cellulase activity for another studies. Thank you so much and have a nice time.

The agar plate method works well. Make sure to use pure cellulose 1%w/w seems to work well. Some methods also add mineral nutrients, but cellulose is the only C source. You can also layer cellulose filter papers on top of agar in a petri dish and add soil crumbs or a suspension to the surface. Disadvantage is that the colonies that develop are less easy to pick off and purify.