Fact: a player who sits in his base and make no effort to scout is a poor player.
Pretty obvious statement you may think, but true. The amount of people that I have played on WOL that just sit inside their base and wait for you is staggering. Scouting is an integral part of the game, without it you don't know what your opponent is doing - which means your chances of winning are slightly higher than zilch. Some Allied players think "Scouting? Nah - I just build a spysat instead". That is an extremely bad attitude to take if you are planning on winning. In my opinion a good player should be able to get by without a spysat. They are an "unnecessary luxury". Yes they are undoubtedly useful, but if a player knows the map that he is playing on, then he should be able to get by without one. If you do not scout, you will not win under most circumstances. If a player scouts out your base and sees that you are just going to sit there and wait, then he could easily build 10 harvesters without you knowing! Resulting in imminent defeat. I know that this is all bread and butter to any good player reading this, but less skilled players have to realise the importance of scouting.
There are a variety of ways to scout. Obviously it is better to scout immediately. One of the reasons for this is that it is far easier to uncover the shroud around an enemies base early on in the game than it is later on in the game (due to there being less units and defensive structures). Another advantage of early scouting is that it allows you to see what type of a player you are playing. You can determine their style of play, be it aggressive, defensive, tech-rush, engineer-rush etc. This allows you adjust your game plan accordingly.
The unit that you should use for scouting, without a doubt, is the dog. I suggest building 2 dogs to scout with (3 or 4 if there are a lot of oil derricks). Approach the enemy's base with these dogs in two different directions. That way one dog at the very least should be able to uncover the shroud over the enemy base. If you are an Allied player then you may consider using 3 GIs to scout as, once deployed, they have the ability to kill dogs. However they are nowhere near as fast or useful in tank battles (mentioned in Section 10. One should also consider using the terror drone if playing on a large map as Soviets. It will easily kill any dogs the enemy uses to scout with.
It is quite simple to scout - but there are a few rules that will help:
In a game lobby look at the map (if it isn't a quickmatch game), look at the red blocks - and remember the position of them.
On a 4 player map, assuming you are playing two player, the opponent always starts in the opposite corner.
Don't waste time uncovering all the map - just the sections en route to the enemy base.
Try your best to uncover the shroud - these quotes, both by very well known military geniuses, are both very relevant:
"To be successful, you must know your enemy as well as you know yourself" - Sun Tzu
"The army that fights in sight of its walls will lose" - Napoleon
If you can see what your enemy is doing then you are far better off. I cannot stress enough how vital it is to scout out an enemy's base. Other units that can be used for scouting include the rocketeer and even the harrier/black eagle. If you fail to scout then you leave yourself vulnerable to engineer rushes and many other tactics.
Summary: If you sit in your base then your chances of success are minimal. Do not be afraid to scout - use any units you have in order to uncover the shroud. You simply have to know what your enemy is going to do in order to combat his actions and in order to plot your own attacks. Scout!