I Looked as Hard as I Could, But I Couldn't See Them

Another task usually performed by my Wonderful Staff is posting the checks and preparing the deposit, which I then take to the bank. Today, after patients were seen and Darling Spouse had gone home (I insisted we come in two cars today so I wouldn't feel guilty staying later to finish things up) I was the one posting the checks. This also included stamping them with the little stamper that says "For Deposit Only." (I love playing with little stampers.)

As everyone knows, checks are no longer printed on plain paper. There are now all kinds of nifty new security devices like water marks and tiny printing and colors. One check had a little pink bar and bore the message, "Bar will disappear when rubbed with finger and reappear if check is genuine." I did, and it did, so it obviously was.

But one that had me stumped said this:

DO NOT ACCEPT IF SECURITY MARK OR INVISIBLE FIBERS ARE ABSENT.

Only one question: how can you tell?