Test in PowerShell code if a folder is a junction point?

If (Get-Item Test Folder).Attributes.ToString().Contains("ReparsePoint"){Code}

Since at least PowerShell v5.0, there is better support for links (or as MS calls them: Reparse Points)

improved Item cmdlets - LinkType property

Linked article is under WMF 5.0 category, which might mean that the method was available since PS v5.0.
These features were included in standard Get-Item, Get-ChildItem, so there are no additional steps required. It can be used on any current PS.

LinkType is a String property on an Object, returned by Get-Item and Get-ChildItem,
it can have one of the following four values: '', 'Junction', 'SymbolicLink', 'HardLink'.

To answer OP's question, you can check if a folder is a junction point using:

if ((Get-Item -Path $Target -Force).LinkType -eq "Junction") { }

To check if a file/folder is a "ReparsePoint", of any kind (Junction, SymbolicLink or HardLink):

if ((Get-Item -Path $Target -Force).LinkType) { }

LinkType value on ordinary file/folder is an empty String, which when used as if condition in PS resolves to False

Get-ChildItem can be used to list all Junction folders:

(Get-ChildItem -Path $Target -Force) | Where-Object { $_.LinkType -eq "Junction" }

Note that value 'SymbolicLink' is the same for both file or folder, so to list only Symbolic links to folders:

(Get-ChildItem -Path $Target -Directory -Force) | Where-Object { $_.LinkType -eq "SymbolicLink" }

Cmdlet Get-ChildItem (alias: dir, ls, gci) now shows ReparsePoint attribute, as l in Mode column, without any extension. But it will not show 'HardLink' and shows l for both Junction and SymbolicLink:

> Get-ChildItem -Path $Target -Force    Directory: C:\UsersMode                LastWriteTime         Length Name                          ----                -------------         ------ ----                          d--hsl       2018-04-12     01:45                All Users                     d-rh--       2018-05-09     06:12                Default                       d--hsl       2018-04-12     01:45                Default User                  d-----       2018-06-24     03:05                Papo                          d-r---       2018-07-27     07:12                Public

• LinkType does not work on folders with special permissions inside \Users and \Users\, even though Get-ChildItem as seen above does work and shows l on them.
• Remove-Item has issues. It can't remove Junction, and if Forced, will remove original contents with it. This is said to be fixed in future version PS v6

Using these improved, or today's standard Cmdlets, has some advantages over previous methods, described in older answers here.

• It does differentiate between Junction and Symbolic Link
  If OP wanted to test if folder is a Junction, checking by Attribute property would result in false positive for folder Symbolic Link.

• detects Hard Link.

• LinkType is [String] as opposed to Attributes property, which is [FileAttributes] type and needs .ToString() or a use of -band

Take a look at this blog:https://web.archive.org/web/20190422210654/https://devblogs.microsoft.com/powershell/viewing-junctions-with-dir/

the way to do it is to copy the built in file system formatting file, modify it so that junctions are indicated, then load it with Update-FormatData:

From the Blog:

The file system formatting rules are in $pshome\FileSystem.Format.ps1xml. I copied this, then in the element [ViewDefinitions –> View –> TableControl –> TableRowEntries –> TableRowEntry –> TableColumnItems –> TableColumnItem] I changed the content of PropertyName with value of 'Mode' to the following:

<ScriptBlock>    "$($_.Mode)$(if($_.Attributes -band [IO.FileAttributes]::ReparsePoint){'J'})" </ScriptBlock> 

This does a bitwise AND on the DirectoryInfo object Attributes property ($_.Attributes) against the .Net System.IO.FileAttributes.ReparsePoint enum value. If the result is not zero, it displays a ‘J’ next to the other file mode attributes. Next, load the new formatting file like this:

 PS> Update-FormatData -PrependPath myFilesystem.format.ps1xml

The PrependPath parameter ensures that the new formatting file is loaded before the built-in formatting files.

Directory alink has a ‘J’ in the mode column, seems to work!

It's in the Mode column J for Junction.