You will find posts in the internet about finding the site column id if the site column is added to content type.
But what if the site column is not added to content type 😦 .
You can find it too 🙂 .
Run the following command lines in the PowerShell :
$web = Get-SPweb http://demo
write-host $web.Fields[“ColumnName”].Id
in demo link put your site URL and in ColumnName put the display name of your site column.
This will return the site column GUID.
hope that help you 🙂 .
Asheshany 