I'm sorry for your tough news. Sounds like the vet said "come back when there are more bad days than good" because he, like you and your family, know that it wouldn't be a good option to put him through treatment, especially given his age. It really doesn't have anything to do with money. Most of the time chemo and radiation for pets is not a good option because that alone diminishes their quality of life. I'm sorry you've been away at school while trying to cope with this. I bet that has been really hard for you. But you have to continue living your life and pursuing your responsibilities, as you are. Your sweet dog would never want to disrupt that. I have a friend who's cat was diagnosed with cancer and the vet prepared her for a few months left, but it's been past that and the cat seems to be doing well and doesn't even act sick. So it's possible they don't really feel it much up to a certain point? I can't say for sure. I also know animals hide their pain better than humans so you're right to be monitoring his behavior closely. That's great to hear he seems to be doing well. Maybe you still have a while left with him :) All you can do is love him with all your heart and be there when you can, which is exactly what you're doing.
To answering your question, from my personal experience and the experience from most of the other people on here, when they start losing their appetite (especially for treats), vomiting regularly, having a lot of 'accidents' on the floor, and/or a lot of diarrhea, it's a pretty clear indicator they're getting close to the end and their bodies are starting to let go. There may be other indicators I don't know about and every animal is unique, but those are some of the common ones to look out for. Again, I'm so, so sorry. <3
Love is eternal & 'death' cannot sever the bond we created. The spirit never dies. The Love, bond, & spirit are ours to cherish always. That is a gift that will never be taken away~