how do i flag an id column based on whether it exists in several other tables?

Use not exists. To bring france rows that do not exist in any of the 3 other tables:

select id
from france f
where not exists (select 1 canada c   where c.id = f.id)
  and not exists (select 1 cameroon c where c.id = f.id)
  and not exists (select 1 gabon g    where g.id = f.id)

If you want to search all tables at once for ids that do not exists in any other, a more generic approach uses union all and aggregation:

select id, min(who) who
from (
    select 'france' who, id from france
    union all select 'canada', id from canada
    union all select 'cameroon', id from cameroon
    union all select 'gabon', id from gabon
) t
group by id
having count(*) = 1

If a single table may contain duplicate id, you can change the having clause to:

having min(who) = max(who)