You can get patches to stick on their rumps, which show when they've been mounted. However, you have to watch and see who's standing for who in order to know which one is bulling. If they don't mount each other, it takes knowing them very well and being able to pick up more subtle signs.
Hillie lets the world know, no problem telling with her, and she also mounts others who are bulling, which is a big help! Plenty is less vocal, and doesn't always mount other cows - but as I am usually milking her when she starts to cycle, I am usually alerted by her behaviour in the parlour.
Another tactic is to watch for the blood coming away a day or two after they've been bulling. Then watch them closely 19 days later for the next one