If you've managed to read through the three previous sections, then you already have a firm grasp of many of the basics of scouting. Moving, not being observed, and picking out a route. This will cover more of the actual duties of a scout, that is, observing the enemy.
From FM 21-100
The smallest security detachment is the scout. The scout is a soldier whose duty it is to see what the enemy is doing without being seen, and to hear the enemy without being heard. The scout must be intelligent, have a strong body, great endurance, keen eyesight, delicate hearing, and excellent memory.
As as scout your commander may use you in all types of combat operations. When your organization is in camp or bivouac, scouts are sent out from the outpost to gain information of the enemy, to prevent his scouts from gaining information of your organization, or both. When your organization is on the march, scouts perform important duties with the advance, flank, and rear guards by discovering hostile troops and promptly sending this information back so that your own commander will not be surprised. In movements by night or dense woods, scouts serve as guides.
As your unit moves forward to the attack, scouts precede it and keep the proper direction for it to follow; they investigate danger areas before the unit crosses them, and select locations where it will be protected from enemy fire. During the progress of the attack they also protect your unit from surprise fire or counterattack by the enemy; they select and occupy firing positions and point out enemy targets.
When their organization is on the defense, scouts serve as lookouts, observers, listeners, and snipers. They may serve as members of patrols to enter enemy lines, both by day and night, to get information of the enemy. They drive off enemy scouts and patrols who are trying to do the same thing.
A trained scout will be able to see and hear things that the average soldier does not. You must be able to pick up in distinct and motionless objects as well as moving ones. Long periods of painstaking search are often required before the position of a hostile soldier is located. As a scout you will conceal yourself as described in the preceding section, but as you will be "on your own" you will have greater freedom of movement.
Scouts usually work in pairs, with each scout having the utmost confidence in the ability of his fellow scout. Train with your partner and make a buddy of him so that each of you know what the other will do under any circumstances. Scouts always work in pairs when scouting in front of their organization in the advance. They move ahead of their organizations as ordered by their commander. Here their duty will be to cause hostile riflemen and machine gunners to open fire and disclose their position, and to overcome resistance from small hostile outposts and patrols.
Remain motionless while observing
Use all available concealment
Observe from the prone position
Expose nothing which glistens
Blend with the background
Stay in shade
Break regular outline of objects
Use extra care when tired. Fatigue leads to carelessness.
Basic Rules of Thumb
At 1,200 yards Infantry can be distinguished from Cavalry
At 1,000 yards a line of men looks like a broad belt
At 600 yards individual files of a squad can be counted
At 400 yards the movements of the arms and legs can be plainly seen
Numbers can be either counted or estimated by the length of time it takes a column to pass a specific pointInfantry produce a low thin cloud of dust while marching
Cavalry produce a hight thin cloud of dust
A broken cloud of dust means artillery or wagon trains
A thick heavy and rapidly moving cloud means automobiles or trucks
Extent and state of a bivouac will indicate the number, strength, and state of an enemy force
Tracks on a road will kind of troops and direction
A freshly made mark will have sharp edges and has signs of moisture which lasts about 15 minutes
A running man digs his toes into the ground while a walking one has fairly even steps
A direction of a vehicle can be determined by the the track of itself over ruts or puddles of water
Speed of a vehicle can be determined by the amount of mud or dirt scattered
Slow moving vehicles leave deep smooth tracks whereas fast moving ones leave cut deeper
Searching Terrain by Day
When observing, observe in a methodical method. Search from left to right in 50 yard strips, passing each time at greater interval, each time overlapping a little of your last search. Don't skip around (you may miss something). Start with the area closest to you and and then sweep outward. If you think you see something, look a little to the side of it, motion can be seen easier out of the corner of your eye. When looking across a body of water when the sun is shining, shield your eyes from BELOW. See figure 67 below.
Searching Terrain by Night
Search the horizon with short jerky movements and short pauses. He should look a little to one side and then to the other to see best on a dark night. Do not use long sweeping movements with long pauses when searching the ground, nor should you look directly at a located object. Look away from flares, flashes of firing, or similar lights, or cover one eye. Eyes will adjust to light and will impair night vision for another half hour.
Messages should be sent orally if writing is impractical, the information consists of one simple idea, or when the likelihood of enemy interception makes it unsafe to send a written one. Messages should be simple and brief, a series of numbers or names should be avoided. The message should be repeated by the messenger as a SOP before he leaves.
The body of a message should be brief, accurate, and clear. Distinguish between facts and opinions. If hearsay information is included, indicate its source. The writer should include all information of value, first about the enemy and then about himself. Information about the enemy should cover:
If it is doubted that the commander received the previous message it should be summarized in the following one. Also indicate the location from which the information is being sent, if it is important. Locate your position by using map reference, reference to specific location, terrain feature, magnetic azimuth from two identifiable features, or azimuth and distance from one point. A sketch or overlay may clarify information contained in message.
Information about the writer should include:
Information is only useful if it is received in time for it to be acted upon. When in doubt, send it at once. If in friendly territory or close to friendly lines one messenger is sufficient. In hostile territory, or when it may be necessary to pass through heavy artillery concentrations, two messengers should be used if they can be spared. They should leave at different times via different routes.
When in danger of being captured, the message should be destroyed. Any further information obtained by a messenger should be delivered at the time the message is delivered. Messengers have right of way and should be given all practicable assistance.