show-trailing-whitespace

show-trailing-whitespace is a variable defined in `C source code'.
Its value is

nil

• Automatically becomes buffer-local when set.
• This variable is safe as a file local variable if its value
  satisfies the predicate `booleanp'.

Documentation:
Non-nil means highlight trailing whitespace.
The face used for trailing whitespace is `trailing-whitespace'.

You can customize this variable.