How to check whether a specific service exists using Powershell?

You can specify the service name using the -Name attribute. By default if it doesn't see a matching service it will give an error. Using -ErrorAction SilentlyContinue you can get an empty variable back.

$service = Get-Service -Name W32Time -ErrorAction SilentlyContinue

Once you have that you can just see if the length is greater than 0.

if ($service.Length -gt 0) {
    # Do cool stuff
    }