[R] lmer - how to test correlations?

[René Mayer]

Dear mixed modelers,
If I have interactions between a categorical
covariate and a random-effects grouping factor.
How can I test formally the correlations?

For example
data(Machines, package = "MEMSS")

fm2aM <- lmer(score ~ Machine + (0 + Machine|Worker), Machines)

Random effects:
  Groups   Name     Variance Std.Dev. Corr
  Worker   MachineA 13.81574 3.71695
           MachineB 61.94506 7.87052  0.805
           MachineC 16.00492 4.00061  0.625 0.772

How do I formally test the significance for every single Corr(A,B);  
Corr(A,C); Corr(B,C)?

thanks for any help,
René