Find out whether celery task exists

Is it possible to find out whether a task with a certain task id exists? When I try to get the status, I will always get pending.

>>> AsyncResult('...').status
'PENDING'

I want to know whether a given task id is a real celery task id and not a random string. I want different results depending on whether there is a valid task for a certain id.

There may have been a valid task in the past with the same id but the results may have been deleted from the backend.