To check the parentheses, brackets, and braces of the current line, press ⇧⌘E (this is the shortcut for User Scripts ▸ More Smile commands ▸ Scripting ▸ Balance).

• if some extra parenthesis, bracket or brace is found, it will get selected
• if some is/are missing, they will be added and selected so that you can check before validating the change.

The ⇧⌘E shortcut also serves the following purposes.

• it will complete a structure for you. For instance if you type if someCondition then you press ⇧⌘E, the structure gets completed into:

  if someCondition then
  
  else
  
  end if

• it will auto-complete a command's syntax. For instance if you select write and you press ⇧⌘E the term will be replaced with its full syntax as shown below.

  write MyAnything for MyDouble starting at MyDouble to MyAnything as MyType

  where the variables names are chosen so as to specify what class is expected.
  Auto-completion of commands works with verbs (commands which belong to an AppleScript dictionary) and with handlers (commands which belong to an AppleScript library), provided the library was indexed - which is the case for the libraries which ship with Smile.