I`ve had dreams were I was Waked and the dream ended when I woke up , not as it just stopped but the dream had a good ending !
How do now when we are getting woke up?
i dunno, but the first dream i remembered since i got here ended by me drifting to the ground laying down in my sleeping position with my eyes closed, the race one finished by me crossing the finish line, and the others just suddenly stopped:p
I think its possibile to be happening many times because your dreams arent separate from you/your brain/your subcounciousness.And above can sense when you wake up either using info from your internal clock or knowledge about what time is your alarm clock on.Or both combined.Or by external sounds you`re not counciously aware of yet.
Your body knows its a wake up time much earlier before you open your eyes.
My dreams are like stories, and I usually wake up before they finish. Conversely, a few times they do finish but keep going and I have this extended ‘happy ending’ phase, which is really fun! Or, if a story-like part of the dream ends and I haven’t woken up, it’ll move on to a new adventure following on from the first. It’s very rarely that the dream ends at the same time the story finishes. For me, anyway.
Well , once I had a PERFECT ! ending of an dreamwho matched the waking up pertfectly .
But I was using autosuggestion though … 
I don’t think your dream knows when to wake up but your body does.
Like when you have an urge to drink some water or have to take a test, go to school and/or work. Your body just becomes accustomed to a scheduled and wonders what it’s doing still sleeping.