On Windows 7, the easiest way to find local user accounts is to use net.exe. Here is a simple PowerShell wrapper called Get-LocalUser:
function Get-LocalUser { $users = net user $users[4..($users.count-3)] -split '\s+' | Where-Object { $_ } }
It returns a list of local users, and you can use all the PowerShell comparison operators. This will check whether a local account "Tobias" exists:
PS> (Get-LocalUser) -contains 'Tobias'
And this line will return all local accounts that contain "test":
PS> (Get-LocalUser) -like '*test*'