My fiancee has PKD and is at 20% gfr. How will we know when it's time to go to the hospital and start dialysis, etc.?

Regular visits. You need to regularly see your family doctor or nephrologist, either of whom can order bloodwork to follow his serum creatinine. If it is over 4.0, most nephrologists will want to start dialysis. Presuming that he is not at that level, now is the time for him to see a surgeon to have an arterio-venous fistula placed to make hemodialysis easier to perform.