For detecting Siegel disc around fixed point in a plot of the Julia set of f one can use average discrete velocity of orbit.
It is decribed here
I have learned this method from Chris King
To highligght boundaries of Siegel disk components in a plot of the Julia set of f one can use this method :
It works for cases when critical point is on the boundary of Siegel disc component.
See : Building blocks for Quadratic Julia sets by : Joachim Grispolakis, John C. Mayer and Lex G. Oversteegen Journal: Trans. Amer. Math. Soc. 351 (1999), 1171-1201
( I can't use image tags )