Personally, I would wait for an affidavit or other information since you cannot determine which checks are supposedly forged. Just because you received an inquiry from an outside source regarding potential fraudulent activity is not an automatic ticket for a SAR in my mind. Do your own internal research based on the "tip" and if you don't have enough to file a SAR, don't file one.
That said, if you obtain further information in the future that does give you enough evidence to file, I would file.
It is actually possible for you and I to have different opinions and yet still live peaceably upon this planet.