For tax professionals, "due diligence" typically refers to qualifying for the Earned Income Credit, Child Tax Credit, Head of Household, and the American Opportunity Credit.  None of that applies to a 1065.

In a general sense, a tax preparer needs to exercise "due diligence" by examining the information that the client gives you to determine if it is reasonable and accurate.  You don't need to audit your clients to get proof of everything, but you need to determine if things are reasonable and make sense.  There is no IRS form for that.