Like you I wondered how I "would know". I found out my

Blizzy had lymphoma in November. She fought a good fight and never really displayed the signs/symptoms of cancer. In the end her enlarged heart (I believe) was her undoing. My daughters birthdays are Dec 23rd and Dec 28th. After Thanksgiving I had a "talk" with Blizzy and made her a deal so to speak. I told her make it to the birthday of the older one (she turned 13 on 12/28) and when Blizzy was ready I'd help her to the bridge. We had TONS more good days than bad from the time of her diagnosis until the day she left. About a week before my little one's birthday (12/23) Blizzy had a few bad days, scared me a lot. I asked her to hold out until Christmas if she couldn't wait until the 28th. Like a trooper my girl made the 12/23 birthday AND on Christmas day she at at the table with us..I fork fed her everything we were eating, she played all morning and all day with my girls and with the extended family that was here for dinner..Blizzy gave out kisses and her famous "hugs" to everyone..later that night she got a bit pale in the gums and my heart and stomach KNEW she was ready. The day after Christmas when she didn't wake me to go out..I knew something was terribly wrong. She had an awful morning..she was very weak and showed signs of breathing troubles. I lay with her and told her..if she was ready to go today, I was ready to fulfill my end of our deal. She picked up her head, and looked at me with eyes that said "ok, Momma, since you have accepted it and will allow me to go..I am ready". She lay her head on my lap and we snuggled. Not long afterward, she started having little seizures and that was it, I rushed her to the vet so she would not suffer not one second. She went peacefully in my arms looking into my eyes.
I promise you, you WILL know, you will feel it in your heart and soul. When the nubby wags are no longer, when she doesn't have that "gleam" in her eye, there are lots of ways our babies show us in their own ways.
I wish you didn't have to go through this. Its a tough thing to deal with. Loving them as much as we do, we don't want them to go, them loving us as much as they do, they fight when they want to give in..for us. But we know in our head, that we cannot let them suffer no matter how much pain their passing will cause us But doing the most unselfish act of ending their pain is what each and everyone of our babies deserve. Sending out positive vibes to you & your fur baby. Kiss her sweet face for me and give yourself a hug. There are so many here who understand EXACTLY what you are going through (although we wish we didn't). Carson sends puppy breath and kisses to your baby.