if observing entangled particles causes the entanglement to break, how do we know that they were ever entangled? I have a working understanding of physics, and am working through quantum entanglement. I read that when we observe one entangled particle, we then know the spin of the other.
I also know that when we observe particles, we break the entanglement. If as soon as we observe the "entangled particle" the entanglement breaks.So how do we know that they were ever related/entangled?
 A: If you measure two entangled polarized photons using polarizing filters that are parallel, then they will always come out the same.
This, of course, requires that you perform the same experiment multiple times. For a single measurement you cannot tell.
Now, this result could be explained by hidden variables, so do a second experiment where the filters are slightly misaligned. The correlations will break Bell's inequality, and hence we have entanglement rather than hidden variables.
A: It is not possible to prove entanglement by measuring a single entangled pair of particles.  Instead, many identically-produced  pairs must be measured to show that there is a correlation between the results of measurements on members of the pairs.  Because of various experimental uncertainties, it is very rare to find that the measurements "always come out the same"; it's more accurate to say that they come out the same more often than would be expected if they are not entangled.
A: Certain special materials/atoms when stimulated (by a photon) will emit 2 photons ( of lower energy of course ) that are of opposite polarization. Thus these photons are related to each other and the word we use is entangled. If you used this material and create photon pairs ( they emit in opposite directions ) they you have entangled photons. Scientists were excited, by measuring/destroying one photon you knew info about the other. Historically anytime we had info on a photon we had destroyed it, the remaining entangled photon is undestroyed yet we know its polarization! 
