Muhammad the easiest, cheapest and fasted way to do so would be by DNA extraction-PCR with specific primers-Gel check
Disign Cladobotryum primers (one day of work) and pay for their synthesis (20 dollars at most for each). Extract DNA with XNAP (sigma) (lvery low cost per extraction), do PCR with the specific primers, review the PCR products in an agarose gel, positive bands will mean an Agaricus strain contaminated, negative bands mean a clean strain. You will need a positive control of Cladobotryum in all your PCRs