If you're going for an all-out rush, then starting on a small map is a necessity. A 1v1 map is ideal since the fog of war is rendered meaningless by such a setup (at least in regards to knowing where to send your minions), but it's much more common to fight in something a bit bigger. Pros will almost always send one of their workers to use as a scout at some point in the early game. The fact that they're willing to remove such a vital part of their economy at such a critical point in the game shows just how important it is to higher level play.

Which worker should be sent to do the scouting? Generally the 2nd or 3rd constructed worker. Zerg have an advantage here since they can also send an overlord to scout a location right off the bat (it may take a while to drag its sluggish brain-butt over there, but hopefully by the time you're ready to send your units in to attack, it will have found what it's looking for). The process runs something like this:

1. Look at the map you're about to play, and count the number of possible starting locations, then look at the number of allies and opponents you have. Open up the map editor to look at the starting locations if you need to.
2. Calculate your odds of picking the right spot to attack. In a 2v2 game with 4 possible start locations, you're looking at a juicy 100%. In a 2v2 game with 8 possible starts, it's more like 33%.
3. If you have a 1-in-3 chance or better, send the worker scout out to do its duty while prepping your attack force. You don't necessarily have to find the enemy before you send the attack units out, you can simply eliminate a possibility or two to stack the odds in your favor. This will keep you competitively fast.
4. If odds are less than 33%, give up on a early rush. Send out that worker scout anyway just to find your opponents and possible expansion areas, but build more economically and develop a more well-rounded early attack force.
5. If you throw everything into a rush and fail to find the right place, you might consider hiding your squad somewhere out in the field. In a minute or two, you'll find out if your opponent was attempting the same thing more successfully. Sending your rush force into his base while he's attacking your soft underbelly is sometimes the only way to deter an attack and save your own butt.

Hope that helps. Learning to scout can really help you improve as a player, and it's not just a method of finding his location. A lot of information can be figured out by dropping in to say 'hello', and maybe even harassing his units. Happy hunting!

Regardless of your race, when playing a none 2 players map, it is risky to do rush build order as your attempt can be easily stopped when your opponent:

• Go for early aggression
• Decide to wall off for economy build

However there are heroic attempts in the past in proleague world:

Boxer's SCV rush in WCG final. 4 players map, when scouted zerg went for fast expansion 3 minutes after game start

http://www.youtube.com/watch?v=Jen46qkZVNI

JulyZerg's drone drill in OSL final. 4 players map, when scouted protoss is walling off.

youtube watch?v=x9pUa_llN98 (can only post 1 link per answer)

(Note: both videos above are in korean, may be a bit confusing but you can see what's going on)