How to Mock method that have other methods call using GO?

I'm trying to write some Unit Tests to an existing code, written in GO, but i really don't know what is the better approach to choose. My method call other methods that call some other methods, and use channels.

func variation(context context.Execution, variationTransfer VariationTransfer){

  numItems = getNumItems( context, variationTransfer )

  validate(variationTransfer)

  mychannel := make(chan interface{}, len(numItems))

  mychannel <- message

}


func validate(variationTransfer VariationTransfer ){

... validation code here

callAnotherMethod()

}

What is the best practice? use Mock or interfaces??