I don't know where to start, but the heat doesn't just wham, here I am, ready for you bull. Each cow will be somewhat different. That is why it is almost an art to be a good heat detector. The cows will vary B/C of the pecking order, B/C of their own personal strength of heat and so on. It is somewhat more of a ritual that includes phases.
1- some cows/heifers start to ball up to a day in advance
2- up to 6-12 hours pre standing heat, they will head but other cattle, but will not stand.
3-standing heat 12-18hours possible.
4- head butting again.
5- go into a calm state of being, or plain old wore out from performing the ritual, THIS IS WHEN TO INSEMENATE.
All of this takes time to watch and learn. There are a lot of "CRUTCHES" to help get cows/heifers bred, but there is no substitute for time spent and a great heat detector. There's nothing wrong with using these crutches either, but when we read about some of the great AI conception rates, I'll promise you that we are hearing from people that are or have a great detector working with or for them. I will have 34 out 36 calves born AI this spring, but that is not all on 1st cycle and it involves a great amount of time. I have used many gomer bulls in the past and absolutely love them, but I don't have near as many cows that I used to have, so can't justify having a gomer bull around. I now have been breeding 1st calf heifers on natural heats and cidring cows, then breeding off of heat detection. I hope that this helps you in knowing some suttle signs of cows/heifers heats. Good Luck,Sincerely, Cab