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.
For more usage examples and a short test, visit this Grammar Monster page.