In current informal usage the words if and whether are used more or less interchangeably, as in these examples from the web:

I Don’t Know If The New Phone Has A Frontal Camera, But It Should

Do you know whether the new iPhone 3 will have the capacity to handle emails . . .

I Don’t Know if Jon and I Will Be Together Next Year . . .

We don’t know them personally so we can’t judge whether they should really be together.

 

☼Most of the time it doesn’t much matter which you use, but sometimes it does.

Use whether if a choice between alternatives is intended.

I don’t know whether we will be together next year. The alternate possibility is that we will not be.

 

☼ Use if to express a condition.

Father will give us the money if we are still together next year.
The money will be given on the condition that the couple are together.