probability of S tag?

[Sahar Ebadi]

Hi,
let say we have this result from the parser: (TOP (S (NP (DT This)) (VP
(VBZ is) (NP (DT a) (JJ good) (NN sentence.)))))
I want to access the "S" tag. and It´s probability. how is it
possible?(JAVA source is preferred)

Thanks!