The difference between the two phrases is that both can be used in the literal sense (to look in the specified direction), "look to" and "look up to" also have a figurative meaning.
The literal meaning:
Look at that, Henry's taking his first steps!
Look to your left and your right before crossing the street!
Look up at the sky, it's Superman!
Look up to your right, there's a green bird at the very top of that tree!
In all of these sentences, "look [x]" is being used to mean "look in the direction specified." This is the literal meaning of the phrases.
However "look to" and "look up to" also have figurative meanings. You can say you "look to" someone to find answers or advice, for example:
In her first few weeks on the job, Monica often looked to her boss for guidance.
This doesn't mean that she literally looked at him, but that she went to him for advice when necessary.
"Look up to" also has a figurative meaning. To look up to someone is to see them as a role model, or to view their behavior as a higher standard which you would like to achieve.
Tommy had always looked up to his baseball coach. He was tough but fair, and taught the team a lot. Tommy had wanted to be just like his coach when he was a kid.
The boy looked her over.
He examined her from head to toe. He could be attracted to her beauty, or looking for an injury after she fell off her bike, or he could be looking for ticks after a walk in the woods.
The boy looked over her.
He looked over her head at something else in the distance.
The boy looked at her.
He could be looking her in the eye, or looking on as she does something, or looking as she does nothing but stand or sit. He turned his gaze towards her.
Best Answer
I think it's basically an idiom that does not follow the normal rules of grammar.
When "look" -- and "stare" -- are used in a phrase with modifiers, we generally drop the "at". "Look him in the eye", "stare him down", "look it over", etc.
I suppose this might be considered somewhat informal, but I wouldn't be surprised to see it in academic writing, etc.