I don't understand why you are not following the good advice given here.
I too chime in to tell you to be honest with the bank. Go talk with your branch manager in private. Tell him what you told us here and of having a concern that this check might not be legit. This way you kind of enlist the bank to be your ally in finding out if the check is real or not.
I also don't understand.
Drew,
You need to have the bank on your side if this turns sour when the check comes back, or the people who sent it to you try to empty your bank account. Be smart about this.