Mockito Verify variables set in a mock before calling a method

I'm trying to write some unit tests around some legacy code and I'm struggling to work out how to test a piece of code. The pattern is basically:

Foo foo = ObjectGetter.getObject(Foo.class);
foo.setVariableX("X");
foo.setVariableY("Y");
foo.performExecute();
//The Foo object has now run some business logic
String bar = foo.getVariableZ();

I have mocked the static ObjectGetter class via PowerMock and managed to get pass in a mocked Foo object. What I want to do now is when performExecute is run check that variable X & Y are set to a predetermined value in Foo but not actually run any code in Foo. Then when getVariableZ is called pass back a certain value.

I can get VariableZ passing back whatever I want but I can't work out how to make that a conditional that variable X and Y are set to certain values! I'm using Mockito and PowerMock. Suffice to say I can't change the code I'm testing for various reasons.