LabModule.ps1

#region LabModule.psd1
@{
    RootModule = 'LabModule.psm1'
    ModuleVersion = '1.0'
    GUID = 'd00578f5-fb6b-4c47-83fb-ed1c8d9a1ec2'
    FunctionsToExport = 'Get-PublicStuff'
}
#endregion


#region LabModule.psm1
# public function
function Get-PublicStuff {
    Get-PrivateStuff
}
# private function
function Get-PrivateStuff {
    'This is very private'
}
#endregion


#region LabModule.tests.ps1
Import-Module .\LabModule.psd1 -Force

Describe "LabModule" {
    Context "Private functions" {
        It "Should invoke private function once" {
            Mock Get-PrivateStuff { 'mocked' } -ModuleName 'LabModule'
            Get-PublicStuff
            Should -Invoke 'Get-PrivateStuff' -Exactly 1 -ModuleName 'LabModule'
        }
    }
}
#endregion