As a link verb it can mean "seem" or "appear" and can be followed by adjectives or nouns.
Right: You look sad, what's the matter.
Wrong: You look sadly, what's the matter.
I looked a real fool when I jumped on the mud.
The kitchen looks a mess.
To talk about a temporary appearance, we can use simple or progressive forms. There is not much difference of meaning.
You look/ You're looking very sad, what's the matter?
It can be followed by like or as if. Progressive forms are not usually used in this case.
He looks like his father.
It looks as if it’s going to flood.
She looks as if she's hungry.
If could also mean "direct one's eyes". In this case it is used with adverbs, not adjectives. Before an object, a preposition is necessary.
Right: The woman looked at me sadly
Wrong: The woman looked at me sad.
A preposition is not used when there is no object.
Right: Look! It's turning red.
Wrong: Look at! It's turning red.
It is used before if or whether. To see, not look.
Right: Could you see if Miki's in the bathroom?
Wrong: Could you look if Miki's in the bathroom?