#Get an alias
$alias = = Get-Item -Path "master:/sitecore/system/Aliases/test"

#Get the link field
[Sitecore.Data.Fields.LinkField]$field = $_.Fields["Linked item"]

# the path to the target item
$field.InternalPath

# The guid if internal. Otherwise this is an Empty GUID
$field.TargetID

# 'internal' or 'external'
$field.LinkType

# Other proporties include MediaPath, QueryString and Target