What is the test for establishing that evidence is 'fruit of the poisonous tree'?

We need not hold that all evidence is 'fruit of the poisonous tree' simply because it would not have come to light but for the illegal actions of the police. Rather, the more apt question in such a case is 'whether, granting establishment of the primary illegality, the evidence to which instant objection is made has been come at by exploitation of that illegality or instead by means sufficiently distinguishable to be purged of the primary taint.'

Wong Sun v. United States, 371 U.S. 471, 487-88, 83 S.Ct. 407, 417, 9 L.Ed.2d 441 (1963).