ruler-mode-show-tab-stops

ruler-mode-show-tab-stops is a variable defined in `ruler-mode.el'.
Its value is

nil

Documentation:
If non-nil the ruler shows tab stop positions.
Also allowing to visually change `tab-stop-list' setting using
and on the ruler to respectively add
or remove a tab stop. M-x ruler-mode-toggle-show-tab-stops or
on the ruler toggles showing/editing of tab stops.

You can customize this variable.