From http://penguincoder.org/pages/A_Slightly_Better_Way_To_Find_Missing_Foreign_Key_Indexes

example output

    add_index :attachments, :parent_id
    add_index :attachments, :asset_id
    add_index :domain_names, :user_id
    add_index :domain_names, :event_id
    add_index :event_memberships, :user_id
    add_index :event_memberships, :event_id
    remove_index :attachments, :parent_id
    remove_index :attachments, :asset_id
    remove_index:domain_names, :user_id
    remove_index:domain_names, :event_id
    remove_index:event_memberships, :user_id
    remove_index:event_memberships, :event_id

potentially_missing_indexes.rb

missing = {}
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
  missing[t] = unindexed unless unindexed.empty?
end

missing.each do |table, columns|
  columns.each do |col|
    puts "    add_index :#{table}, :#{col}"
  end
end
missing.each do |table, columns|
  columns.each do |col|
    puts "    remove_index :#{table}, :#{col}"
  end
end

Found the article back in the days with archive.org. It only contains the script. https://web.archive.org/web/20090928080237/http://penguincoder.org/pages/A_Slightly_Better_Way_To_Find_Missing_Foreign_Key_Indexes

A further article is linked there:
https://tomafro.net/2009/09/quickly-list-missing-foreign-key-indexes