Two main cases come to mind:
1) If you are saying that you need to look at it in order to be able to make a decision or in order to be able to tell you more about it. For example:
A: My sweater has a stain, do you know how I can remove it?
B: I don't know, I would have to look at it.
(That means that there is a need to look at it to be able to decide how to remove the stain.)
2) As part of a 'second conditional' statement - see this webpage:
The 'second conditional' is the third one in the list.
There are four main kinds of conditional statements in English, as well as some mixed conditionals which are a combination of the basic four. What is called a 'second conditional' is the case that you are mentioning:
Its basic structure is: if + past simple, ... would + infinitive
It is used for the theoretical unrealistic present or future.
"I don't have a car, but if I HAD one I WOULD DRIVE you to work every day."
"If we WERE younger we WOULD COME with you to the show tomorrow, but we can't, because it is for teenagers only."
Note, that even though the past is used ('had', 'were') it means in the present and/or future, not in the past.
For the past (that is, for impossible-to-change situations of the past) you use the "third conditional", the fourth one in the list in the website.
For example "Yesterday wanted to come to see you, but I wasn't able, because I missed the bus. If I HADN'T MISSED the bus I WOULD HAVE COME." (Here the second sentence is a 'third conditional')
This third conditional, for the impossible-to-change past, follows this pattern:
if + past perfect, ... would + have + past participle