whether or not
When or not is needed after whether.
Drop or not when it is redundant: Tell me whether the truck left. Keep or not only when the sentence needs the negative alternative, usually when you mean regardless of the outcome: The load goes out whether or not the truck leaves.
Examples
Use Tell me whether the truck left. (or not adds nothing)
Use The load goes out whether or not the truck leaves. (needed: regardless)
Avoid Tell me whether or not the truck left. (or not is redundant)