polemoscope. An opera or field glass with an oblique mirror arranged for seeing objects not directly before the eye. “The detective used a polemoscope to observe the suspect unobtrusively

This device is usually a small monocular with a prism,
so that the user appears to be looking away from the
suspect, to avoid alerting suspect. They used to be
advertised in pulp magazines, called "See-backoscope".