That's what a Hunt command combined with a target arc is supposed to do. From the manual:
"Hunt - Infantry - this command maximizes the unit’s awareness for possible
enemy contact. Soldiers are advancing slowly, weapons
ready. Upon seeing any enemy units, the unit stops immediately.
This is a good command to use when enemy contact is
imminent. In combination with a Target Arc command, Hunt is
restricted to only the area within the arc, and ignores enemy
units from outside the arc."