Do you mean sims in your household, or sims that your sim knows but doesn't live with?
If it's a sim you controlled or have an urn or grave for, it's like Twinmum said.
If it's a sim from an inactive household that did not die on your sim's lot, or a community lot that your sim was on at the time of death, then there is no tombstone or urn for them as far as I know. They are just gone and you don't get to see their ghosts.