My experience was that male dogs spotted it first, and the girls became flirty at times, or moody. When it really gets going, you will notice her licking her bottom clean, as the pink discharge occurs. We got ours neutered before the second season, no problems in later life at all. Using kennels can be well nigh impossible, my son was lucky that he found one with an isolation block and separate field. Otherwise it’s not fair on male dogs , and they will bark and misbehave. Other dog owners won’t like it.
We didn’t know our girl was going on heat till an Alsatian ran into our house, and into the bedroom where OH was having his early morning tea in bed. Next day there were three sheepdogs sitting outside.