factories_spec.rb

FactoryGirl.factories.map(&:name).each do |factory_name|
  describe "The #{factory_name} factory" do
    it 'is valid' do
      build(factory_name).should be_valid
    end
  end
end

my_model_spec.rb

# spec/models/my_model_spec.rb

describe MyModel do
  it 'has a valid Factory' do
    build(:my_model).should be_valid
  end
end

Rakefile

desc 'Run factory specs.'
RSpec::Core::RakeTask.new(:factory_specs) do |t|
  t.pattern = './spec/factories_spec.rb'
end

task spec: :factory_specs