You can easily check the local admins with the compmgmt.msc or direct via cmd “compmgmt.msc /computer:$ComputerName”

If you want to achive the Task with powershell, you just Need to use this code:

#region Get-LocalAdmins
function get-localadmins{
[cmdletbinding()]
Param(
[string]$computerName
)
$group = get-wmiobject win32_group -ComputerName $computerName -Filter “LocalAccount=True AND SID=’S-1-5-32-544′”
$query = “GroupComponent = `”Win32_Group.Domain=’$($group.domain)’`,Name=’$($group.name)’`””
$list = Get-WmiObject win32_groupuser -ComputerName $computerName -Filter $query
$list | %{$_.PartComponent} | % {$_.substring($_.lastindexof(“Domain=”) + 7).replace(“`”,Name=`””,“\”)}
}
#endregion Get-LocalAdmins

Now you can query every Computer with “get-localadmins computername” – You need local admin rights to do this.