How to tell if your Shimano HG53 bicycle chain is stretched/worn out and needs to be replaced

In this video we are taking a look at chain stretch and wear. Both of these factors affect how your gears work, if the gears skip and how the bike feels overall. By regularly oiling and checking your bicycle chain, you make sure that the chain does not wear out prematurely.
I am using a 9 speed Shimano HG53 chain, Shimano HG50 rear cassette on a Shimano SLX M675 rear centerlock hub. The front chainrings have been replaced from 36 tooth original Sram chainrings to Shimano Deore 32 tooth chainring (can't find the reference code).

[@wacio]

I have 8000km on this type chain I bet I can get another 8k before it needs replacing. I don't know why everyone keeps recommending replacing serviceable parts. I have masterlink to do quick fix on the road (but never broke chain before). I'm replacing chains when drive becomes granular or noisy while riding.