The short guide you never knew you needed.
Greetings my fellow Legionnaires, hope you are enjoying this game, like I am, and I bet you're wondering, "This game hasn't even been out for a week yet, and you're already giving me tips on peeking with portals, give me a break.", and for that I have to say...
Well played, you are right, but I wanted to "patent" portal-peeking, so made this guide to do so, while attempting to help you.
So let's get to it.
First Section - Basic Peeking
Now I'm no expert on proper peeking etiquette, but the basics of it are, shoot your target, minimise your chances of being shot, really simple, in theory, and nearly every game has it's own way that you should peek, due to it's inherent mechanics, but since this is a faster pasted game, the "adadad" spam can be effective, but due to inaccuracy while moving not existing at all, staying mobile and tracking your opponent(s) is much better.
Second Section - Portal Etiquette
This is how you should or shouldn't use teammate's portals, such as spraying randomly through them, rushing through them, or placing your portals nearly right next to his, and cause the enemies to have several firing lines on your position.
Equally, if you notice one of your teammates run through one of your portals, don't immediately remove the one on their end if you can help it, as they may have made a giant mistake, and wish to perform a take back on that action.
This is mainly just being aware of your teammates, ignore this if you play custom or social matchmaking.
Third Section - Portal + Peek = Sick Angles
If you are at all an intellectual, you will be aware of your ability to see through your own portals, which can be advantageous for the titular PORTAL PEEK, and can feel satisfying to completely bamboozle an enemy, who has no idea where you're firing from.
The best ways to take advantage of this, is to avoid easily spotted portal locations, so instead of firing them on the walls, where the average player may be looking, attempt to get them on slopes, ceilings and floors, ceilings being the better of the three, but be aware that the enemy are still capable of firing through your portals, so avoid being fragged through your own portals, as it kind of instils some deep-seated depression in your inner-self.