16+ year old siam dying

Question
My cat old siam is in the final stages; I want to let him go on his own (no euthanasia)   How do I know when the very end is near; what can I do for him? He's on his side, breathing but twitching. He's emaciated but does not seem in pain.

Answer
Elaine,
Your cat should be humanely put down now.  It is cruel to let him live just because you think he should "go on his own".  What you are doing is very selfish and clearly not the best thing for your cat.  If you love your cat, take him to a Veterinarian and let them quietly put him down. This is the most humane and loving thing you can do for him.  
Becky