Simple - use the key combination Ctrl-K followed by the two-letter digraph code directly in insert mode.
For example, Ctrl-K DE generates ∆ and Ctrl-K 23 generates ⅔.
For a list of all the available digraphs and their key codes, use the command ":dig".
|Sample Digraph Codes|
" Show tabs and trailing spaces.
" Ctrl-K >> for »
" Ctrl-K .M for ·
" (use :dig for list of digraphs)
set list listchars=tab:»»,trail:·
Here's what it looks like:
|Using Digraphs to see Whitespace Characters|