Test-Path is your friend whenever you want to check whether a file or folder exists:
Test-Path C:autoexec.bat
Test-Path C:windows
Test-Path C:windows
Test-Path can check a lot of other things as well, though. It accepts any PowerShell drive, and as you may know, in PowerShell drives are not restricted to the filesystem.
To check whether an environment variable exists, use this:
Test-Path env:computername
To check for an alias, use this:
Test-Path alias:dir
You can even check for registry keys like this:
Test-Path hkcu:softwareidera