example output

attachments: parent_id, asset_id
domain_names: organisation_id
event_memberships: user_id, event_id
events: editor_id
group_actions: user_id, group_id
groups: user_id
icons: parent_id
invitations: sender_id
legacy_actions: item_upon_id
news_items: author_id
organisations: midas_id
pages: author_id
pending_event_memberships: invitation_id, event_id
resources: user_id, resourceable_id
subscriptions: subscribable_id, user_id
taggings: tag_id, taggable_id, user_id

potentially_missing_indexes.rb

c = ActiveRecord::Base.connection
c.tables.collect do |t|  
  columns = c.columns(t).collect(&:name).select {|x| x.ends_with?("_id" || x.ends_with("_type"))}
  indexed_columns = c.indexes(t).collect(&:columns).flatten.uniq
  unindexed = columns - indexed_columns
  unless unindexed.empty?
    puts "#{t}: #{unindexed.join(", ")}"
  end
end

Thanks for the tip. May i suggest for lines 2-3:
c.tables.collect do |t, sfx=['id','type']|
columns = c.columns(t).collect(&:name).select { |x| (s=x.split('_')[1]) && sfx.include?(s) }