Euthanasia

A depressing topic for a Saturday afternoon but I’ve been thinking about it, a lot, recently.
Sydney, our family cat isn’t well.  He’s losing weight and he’s increasingly lethargic.  The vet has prescribed steroids and she thinks the problem is a tumour that lines the intestine. 
I can’t talk to Syd to ask him whether he’s in pain.  I can’t ask him whether he’s enjoying life.
My internet research hasn’t really helped me, especially when I read that sometimes a cat will purr not through contentment but because it distracts the cat from the discomfort it’s experiencing.
I could get all sentimental about how Syd has always been there and how he’s been a part of our life for so long but that doesn’t help.  Basically I need to know whether now is the right time to “put him to death” as Ethan puts it.
The advice I’ve received so far is “Don’t leave it too long” which doesn’t really help because I don’t know when “too long” is.
And it’s not really my decision either, it’s a family choice.
The children understand death if it just happens but I think they’ll struggle with making a decision that will bring it forward.
I see my husband getting upset whenever we start to talk about it and I struggle to remain composed too.
I don’t know what to do, or who to ask.