You'll know when Dawnguard starts: a guy literally approaches you and tells you to join up.
Dragonborn starts after you've completed 'The Way of the Voice', I think. You'll come upon some Cultists of Miraak traveling across Skyrim.
Hearthfire starts when you receive a letter from the Jarl, though you can pretty much start it manually by going to talk to one of them.
Can you explain? I'll gladly read spoiler tags. I don't wanna miss out on a bunch of stuff and in all the time I spent with OG Skyrim I didn't play dawnguard more than a few hours.
Dawnguard kicks off around level 10 and randomly spawns vampires that can randomly attack citizens, depriving you of valuable people like shopkeepers.
I used to play the PC version with a mod that would only kick off the Dawnguard questline once I'd beaten "Dragonslayer", but that mod doesn't seem to exist yet for the SE.