I apologize in advance if this has been asked many before, but I didn't see anything that answered my question on the first page of threads. Take, for example, an isolated subatomic particle which breaks down into two entangled particles. From what I've read, each of these two particles are in a state of both having an "up" and "down" spin state simultaneously. They lose this property when one is measured, and one will take on either a definite "up" or "down" spin state, and the other particle will take on the opposite spin state of the first. My question revolves around the idea of "having both up and down" spin before measurement. How are we able to detect that a particle shows signs of having both an "up and down" spin state, without directly measuring the particle in such a way that it takes on a definite spin state? Similarly, how do we know one of those particles wasn't -always- in an "up" spin state, rather than some sort of "up and down state at the same time, until measured"?