how to find out if tenant has left after section 21

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I have served S21 on my tenant. He does not respond to e mails or telephone calls. How would I know if he has left on the date he should leave which is 15th July?. I know I am not supposed to open the door and go in. As its a block of flats and we hardly see anyone around when ever we go that way its not as though we can ask a neighbour either. therefore how do I know whether we have to proceed to the next stage or not?. I understand that RLA does deal with court proceedings for the next step for a very reasonable fee and wonder why its not posted as a service provided with the list of benefits as i am sure many of us could use that facility. Rainbows