Depending on the reason for wanting to make someone look like he's just been assaulted, the formula for looking beat up can vary in degrees. Ripped clothing, red skin, tousled hair and even fake blood can all give an onlooker, audience or good friend the impression that you're after.
Decide what event you're trying to recreate. Being beat up by a gang is different than a girlfriend slapping you in the face. Determine the level of assault and the storyline. Let this be your compass as you try to make things look authentic.
Rip pieces of clothing by putting the shirt or pants on and pulling them in a realistic manner. Simply cutting them with scissors doesn't look realistic upon close inspection. A rip is messy and tends to happen along the seams as the fabric comes under duress.
Find a way to make the face, neck or other strategic areas red. If you want to look like you were just slapped, you'll need to have someone actually slap you. Ice can also make an area look very red temporarily.
Mess up your hair. Every good whooping makes a mess of your hairdo.
Apply fake blood to the supposed area of injury. See the eHow article How to Make and Use Fake Blood" for a recipe to make at home, or buy fake blood at a costume or theatrical shop. Read all instructions carefully so as not to damage the clothing permanently.
Act injured. Appearing out of breath, upset, angry or even crying makes the show complete. Acting stunned and sitting still as if in shock is also a great way to look like you've been beat up.
Remember the amount of injury needs to match the story and the time frame of when it happened. Swelling and bruising take some time to happen.
Blush lasts too long for realistic skin redness, so unless you're filming it, don't use makeup. Avoid overacting.