Mid and Late Game Scouting
In longer games, it is necessary to scout the enemy every now and then, to see what he is up to, where he is expanding, what unit composition he uses, the level of his weapons and armor upgrades, defence lines, and possible weak spots in his defences.
At this stage in the game, it is rather difficult to scout with a single worker, because the enemy will stop you in no time. Luckily, all races have advanced scouts tuned for the mid and late game
If you are a Protoss player, scouting can be performed with an observer. If you do not have observers yet (you should get them) try using a phoenix, either a real one or a hallucination. Their high speed and long sight rage makes them perfect for scouting. Once you have observers, move them in key positions around the map, in the enemy base, or even task them to follow the enemy army. Remember, observers are invisible unless the enemy has detection.
If you are a Terran player, you can use scans from Orbital Command, marine drops (do some damage while you are at it), or a hellion run by. As a means of detection, you have the Raven, a supporting spell caster from the Starport, and scans. You can also use vikings.
If you are zerg player, you should use your overlords and creep. During the game, you will create lots and lots and lots of overlords. Don't keep them all in one place, spread them across the map, in key positions. Spreading creep is also advised. Your units will move a lot faster on creep, and you will be able to see what is happening on your creep network. Zerglings with speed upgrades, mutalisks and burrowed roaches are also effective at scouting.