Marino's Mantles stick out like a sore thumb once you identify the "look."
For example, note the straight up and down lines and lack of any slant. His Ms have pointy tops and an almost perfectly vertical post, which is completely atypical.
Here is another Marino Mantle for comparison:
http://www.operationbullpen.com/gallery1.html
He got a little better later in his forging career, but still left his fingerprints making them easily identifiable.