The real "detective" is the "boss" who directs them.
The reader can easily see that in all cases where a crime, such as forgery, is concerned, once the identity of the criminal is ascertained, half the work (or more than half) is done.

The agencies know the face and record of practically every man who ever flew a bit of bad paper in the United States, in England, or on the Continent.

If an old hand gets out of prison his movements are watched until it is obvious that he does not intend to resort to his old tricks.

After the criminal is known or "located," the "trailing" begins and his "connections" are carefully studied.

This may or may not require what might be called real detective work; that is to say, work requiring superior power of deducing conclusions from first-hand information, coupled with unusual skill in acting upon them.