What are the earliest signs of coming into season? We have two Jack Russells that are house pets. Male 5 years, female 2 years.

During her first season, I boarded the male the moment he became interested in her. A week later she showed clear signs, swelling, redness, etc. At her second season, I boarded him again, but must have been a few days late because she delivered their first litter of puppies on the 12th June. She showed no visible signs of being on heat, yet obviously had been. She is due to come into season again soon. I dont want them to breed again until late next year so need to watch her carefully for the earliest signs so that I can board the male. What is the first thing I should watch for?