How to know if a fish died because it was it s time to die?

[Dorange]

If you know what I mean?
It doesn't matter how perfect is the conditions in the fish tank, fish won't live forever, so how can you tell it died because it was just too old and it was its time to pass to the other side?

How old was it and what is their usual lifespan? I've omly had my fish a year so none has died of old age yet. A lot of fish, if kept in the correct conditions, can live for 5 year +.

It is a massive goldfish that was given to me at the beginning of the year.
I was told he is ± 10 year old.
He came in a unsuitable tank but I moved to a 240l and he lives there in perfect water conditions with 5 zebra danios.
He is fed flakes (the only food he has ever had before coming to me) and now he accepts sinking pellets, frozen and dried bloodworms.
I tried peas/broccoli/lettuce but I am not sure he ever nibbled.
He used to swim about happily in his new tank but now is behaving exactly as he was while in the old one. Bottom sitting and looking depressed...
No signs of disease and the other fish are happy.

It might be that he suffered some damage from being kept in unsuitable conditions. There's nothing you can do about the past, it sounds like his living conditions now are brilliant. Is he pooping OK?

I need to sit down and observe his pooping routine better...I havent got a clue...

If/when he does poo, have a look at it and see if you can see any bubbles or if it looks a bit white and stringy.

What would white and stringy indicate?