I am so sorry to read about Jarvis, my heart goes out to you
I lost my Harley boy very suddenly to this last year, his tumour wasn't found until too late so he had no chance of treatment.
I worried too whether I would know as he was still so lively and well, but I assure you you will. Don't ask me how, I think it's just there in that special bond we have with them.
Enjoy your time with Jarvis.
We are here for you