Onan Genset Spark Arrestor Cleaning

Onan 8kw, 1800 hrs. Followed directions to clean spark arrestor which may or may not have ever been done by PO. Question is this, when running for 5 minutes under load with clean out plug removed, did not observe any soot being expelled or any different exhaust sound than when running with it installed. How do I know this procedure was successful? Could the spark arrestor be so plugged up that removing the clean out does not achieve the expected expelling of soot?
Unit runs fine, just trying to be sure PM is up to date.