[...] if is used to express a conditional, whether to introduce alternative possibilities. [...] If you can coherently insert an "[or not]" after either the conjunction or the clause it introduces, you need whether. Examples: "He didn't know whether [or not] it would rain"; "She asked me straight out whether I was a fetishist [or not]"; "We told him to call if [or not? no] he needed a ride [or not? no]." [...]