I'm really sorry about your dear cat. :(
I do know how you feel. I obsessively adored my 14-year-old cat who died from Stage IV lymphoma last November. (I wrote about it on this website and got lots of helpful advice that you may also like to read, you can find the thread if you search my username.) I was so upset about her diagnosis, her decline, and her death. Looking at photos of her now, or thinking too much about her, will still cause me to start crying and feeling terrible that she has died. But I have accepted that this is an inevitable and natural stage in one's relationship with every beautiful and cherished pet. I have also accepted that I did the right thing in not choosing to treat her cancer aggressively. Hopefully you can come to these acceptances too, over time.
Considering that we cannot protect our pets from death, as good pet owners we can do our best to ensure that our pets are given a dignified, loving, and gentle death, rather than a painful, scary, or protracted one. That is why I personally would not choose to operate on, or give chemotherapy to, a geriatric cat with cancer. When my cat was sick and I was agonising over what to do regarding treatment, I was told so many stories about people who chose to aggressively treat their pets who were terminally ill, and the pets died alone at the vets, or on the operating table, or due to a complication of the treatment (such as chemotherapy). These people had huge regrets. I have no regrets, apart from the fact that my cat had to die. :( At the time, though, I felt that I had chosen to murder my cat, and you may feel that way too initially, but I know that you will know - as I knew - when it is right to protect your cat from further suffering through euthanasia, and at that point death cannot be avoided, it is a matter of how it happens. With time, you accept that too. Be wary of vets who push aggressive treatments when there is a significant risk your cat would not cope, a good vet would not push such treatments in such a situation. I know none of my vets (I consulted three) did - they told me the options, but all three seemed to prefer the option of doing nothing/treating palliatively.
You don't have to worry about how you will know, you will truly know. I worried a lot about this too, but there comes a point where you feel it is cruel to allow your cat to suffer or decline further. You may feel guilty and desperately worried that she may die before the vet can see her. Usual signs include hiding away, not showing joy in your presence or purring when you pat her, disinterest in eating or drinking, lack of personal dignity/hygiene in the way that your cat usually carries herself, appears in discomfort or pain, obvious physical signs of the cancer, etc. My cat did swell up with fluid - lymphodoema, a result of her cancer. Ascites is similar and can also be a result of cancer, I don't know if pets get it but I imagine they do. It may not happen for your cat though.
I'm sorry to be all doom and gloom, just trying to be helpful by giving information, let me know if I shouldn't because it's too upsetting for you. On a positive note, enjoy every minute with your cat, don't obsess over different things being a sign of cancer - I did, it's not helpful and doesn't change anything - and remember that death is a natural stage of life. And your cat's life has been full and she has been utterly loved - the best life a cat can hope for! You don't know how long your cat has left, she could have months/years and you don't want to spoil that time by worrying.