I want to do a custom search, e.g question I cast a closure, and contains a keyword, but I don't know any tags other than
user:id, where can I find them ?
They're on the Search Options page, under "Advanced Super Ninja Search Options". The easiest way to get there is to do an empty search (just hit Enter in the search field)
In your case, there is no field for "question I cast a closure"; the closest is
closed:1 to match questions that are currently closed