For girls I would say anything either revealing or ostentatious would be an attempt to show off either their physicality or their economic status. With guys I would say someone wearing Under Armour compression gear or Nike ProCombat style clothing would be trying to show off.
I think any time teenagers where clothing that does not fit, it is in an attempt to draw attention to themselves. This goes for clothes that are too tight or clothes that are far too big. Granted, many girls (and guys) try to "hide" in oversized clothing, but even then, I think in some way, it is a cry for attention.
I think this is a very interesting question. The first poster hit the nail right on the head that the key is about dressing appropriately for all the given factors. Everyone needs to consider their appearance, age, and the situations they are going to be in during the day before they dress in the morning, or purchase clothing. Not all clothes are ok for all ages or sizes or places. I personally find short shorts offensive anywhere but at the beach or maybe the park. As for trying to show off, I don't think that any particular clothing is always a sign of showing off. You can tell by how a person wears their clothes and where they wear them too. Wearing clothes that show a lot of skin to places where that is inappropriate (like school or church or your job) is likely a way to show off. Wearing clothes that are inappropriate for the situation is a good sign. Sometimes someone wants to show off that something is very expensive or has certain label, even though those clothes are uneccessary or inappropriate. I once knew a woman who wore her fur coat to every occasion (whether it was cold or not). Clearly she wanted everyone to see that she owned an expensive fur. If it seems like someone constantly mentions their clothes or appearance or seems to be wearing clothes that are out of place likely the purpose is to "show off".
For guys, the only "show off" clothing that I can think of would be muscle shirts. Any guy who is wearing a tight shirt with no sleeves in public probably thinks that he's pretty buff. Outside of that, there's not a lot of room for that sort of thing for a guy.
This promises to be an interesting discussion! I think that the main thing that teens don't understand about their clothing choices is that outfits need to be appropriate for the place and activity where they are worn. For instance, short shorts and crop tops may be fine at any age if you are spending the day at the beach, but would never be acceptable for most workplaces. It seems to me that many young people try to push the envelope by wearing clothing in the wrong venue; I see students wear their dance club clothes to school, go out shopping in their bathing suits, and so on.
As far as having certain clothes work for a certain physical appearance, that is more a matter of buying the correct size than anything else. Girls seem to prefer clothing that is two sizes to small, while guys tend to buy things two sizes too large. Matching the clothing size to your actual physique is usually the best bet.