Just that really - what little clues suggested to you the season was coming? We took on a unspayed 2-year old recently and she is due soon. My previous dogs had all been males so this is new to me.
(And, for the benefit of any posters who may get snarky about the fact we have an unspayed dog, we do plan on getting her spayed once her season is finished. But felt it would be kinder to give her a proper settling in period in our house before hoiking her off to the vet for major surgery! So no judgement please.)