Isn't disentanglement a form of transmission of information? I.e. if suddenly my particle disentangles, I can assume it's been manipulated? (which is a binary state of tangled/untangles (0,1))
And how can you tell if the particles are no longer entangled without observing both? It's not like there's some outward, observable property of the particle that says "I am entangled". It's simply a statistical correlation between the states of the two particles.
So, the statistical correlation between states of particles is literally the fundamental thing that defines quantum entanglement.
Suppose you and I both get a coin and we flip them. You then turn over your hand and reveal heads. If the two coins are "entangled", there is now a greater than 50% probability that my coin will be tails, which would not be the case if the coins weren't entangled.
Quantum entanglement works essentially the same way. "Flipping the coin" is equivalent to observing some state of the particle, like spin. In both cases, entanglement is evidenced by the correlation between the states, nothing more.
Now, imagine we had entangled coins and we were in separate rooms. We flip our coins and you observe yours, noting a heads. In my room, I can't know what you did or didn't observe. So when I observe my coin and see tails, I don't realize it's a consequence of your observation. From each of our individual perspectives the coin flip looks perfectly random. This is why information transfer isn't possible through quantum entanglement.
The correlation is that you know from your reading what the other person will detect. For example, if you detect a spin up particle, they will detect spin down. But you don't know when they have, or if they have, done the detection.
You would know when you have two particles entangled at the moment of entanglement. You could know you have a half of a pair of entangled particles when the other half is sent away. But you could never know when the pair has become disentangled.
