Are you talking about cleared checks, or deposited checks?
If you mean that suspicous items have cleared the account, then you need to confirm whether they were authorized by your customer. If the customer did not authorize them, then you should obtain a forgery affidavit and can then file a report with the police for the bank's loss.
If you mean that suspicious items have been deposited and/or returned against the account, you need to verify with the maker and/or paying bank whether the items are in fact fraudulent. Then you can decide how to approach your customer and what information you should report to law enforcement.
In either case, if you are not able to confirm that the items themselves are fraudulent, but still feel that the activity is suspicious, consider filing a SAR (if the amount involved meets the reporting threshold).
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To avoid criticism do nothing, say nothing, be nothing. ~Elbert Hubbard