Test-VirtualDirectory function will help you to find out whether virtual directory exists in IIS from powershell script

virtual_directory_exists.ps1

Import-Module WebAdministration

function Test-VirtualDirectory
{
    param(
        [string]$WebsiteName,
        [string]$VirtualDirectoryPath
    )

    if (-not (Test-Path "IIS:\Sites\$WebsiteName")) {
        throw "$WebsiteName website could not be found in IIS"
    }

    return (Get-ChildItem "IIS:\Sites\$WebsiteName" | Where { $_.Schema.Name -eq 'VirtualDirectory' -and $_.Name -eq $VirtualDirectoryPath } | Measure).Count -eq 1
}