Mere trailing is often simple, yet sometimes very difficult.

A great deal depends on the operator's own peculiar information as to his man's habits, haunts, and associates.

It is very hard to say in most cases just where mere knowledge ends and detective work proper begins.

As for disguises, they are almost unknown, except such as are necessary to enable an operator to join a gang where his quarry may be working and "rope" him into a confession.
Detective agencies of the first-class are engaged principally in clean-cut criminal work, such as guarding banks from forgers and "yeggmen"-- an original and dangerous variety of burglar peculiar to the United States and Canada.

In other words, they have large associations of clients who need more protection than the regular police can give them, and whose interest it is that the criminal shall not only be driven out of town, but run down (wherever he may be), captured, and put out of the way for as long a time as possible.
The work done for private individuals is no less important and effective, but it is secondary to the other.