ActiveJob::Base.queue_adapter.enqueued_jobs

[
  {
    :job: ExampleJob,
    :args: # whatever args,
    :queue: name_of_the_queue
  },{
    :job: AnotherExampleJob,
    :args: # whatever args,
    :queue: name_of_another_queue
  }
]

ActionMailer::DeliveryJob.queue_adapter.enqueued_jobs.count{ |x| x[:job] == ActionMailer::DeliveryJob}

# In Rspec, for example,
expect(ActionMailer::DeliveryJob.queue_adapter.enqueued_jobs.count{ |x| x[:job] == ActionMailer::DeliveryJob}).to eq(1) # expecting 1 async email