Michelle,
I have to assume that you have a file copy of the SAR and your actual concern is verifying FinCEN's receipt of the original rather than obtaining a copy.
Either way, your only hope for verifying its receipt or obtaining a copy is the FinCEN helpline, 800-800-2877. It's an understatement to say you will have to be persistent in working your way through the voice mail decision tree. If your repeated efforts to talk to a human being and receive a confirmation are unrewarded, send a copy of the SAR "return receipt requested" as Jules suggests. Include a very brief cover letter indicating it may be a duplicate filing.
When the examiners come, they will know whether the SAR was received in Detroit - you need to be certain that it was.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.