Whether or If
Whether is a conjunction that precedes the question in the reported speech. If is used in similar manner, but there are a few differences.
- I'm not sure if/ whether she has heard the news.
- If  you don't know if/ whether you use proxy server when connecting to the Internet, click on the detect button [..]
In the second sentence, the first if cannot be replaced with whether because whether is not a conditional conjunction.
If there's an alternative marked by or, then whether is the more often used conjunction (note: in the second example it's less correct to choose whether but in the third one it is altogether wrong).
- I don't know whether/ if she believed us or whether/ if she just pretended to.
- I don't know whether she believed us or not.
- I don't know whether or not she liked the idea.
- I don't know if or not she liked the idea.