How do I test to see if a user has access on a site?

Question

I am completely new to PowerShell, but I have a requirement to build a PowerShell script that will run against a site, a set of sites or a web application. How do I add a check within my script to test if a user has rights on the site they are running it
on or not? Am clueless on how to test if the user has rights to a site....I basically am trying to test, then let them know that they do not have access on a site or not. 
How can I get this done? Please help.

Answer 1

I now have this code which I think should do the trick, but I am getting an exception when i step through it. The $serverContext variable is always null. Does anyone know why I am getting this?
write-host "Please enter the url of the site collection"
$url = read-host
write-host "Please enter the username of the user"
$userName = read-host
$site = New-Object Microsoft.SharePoint.SPSite($url)
$serverContext = [Microsoft.Office.Server.ServerContext]::GetContext($site)
$userProfileManager = New-Object Microsoft.Office.Server.UserProfiles.UserProfileManager($serverContext)
$userProfile = $userProfileManager.GetUserProfile($userName)
$userLogin = $userProfile[[Microsoft.Office.Server.UserProfiles.PropertyConstants]::AccountName].Value.ToString()
$webs = $site.AllWebs
foreach ($web in $webs)
$permissionInfo = $web.GetUserEffectivePermissionInfo($userLogin)
$roles = $permissionInfo.RoleAssignments
write-host "Now checking the permissions of the user " $userLogin " " "in the site " $web.Url
for ($i = 0; $i -lt $roles.Count; $i++)
$bRoles = $roles[$i].RoleDefinitionBindings
foreach ($roleDefinition in $bRoles)
if ($roles[$i].Member.ToString().Contains('\'))
write-host "The User " $userLogin " has direct permissions " $roleDefinition.Name
else
write-host "The User " $userLogin " has permissions " $roleDefinition.Name " given via " $roles[$i].Member.ToString()
Thanks for the help.