There may be some new Spectre feature that lets you direclty extract the phase modulation but I can think of two other ways that might work.

1. You could use a phase detector. One way or another you would have to filter a ripple component. You might even try a complete PLL. The filtered phase detector output should track the modulation if the filter is properly designed.

2. IF you have SpectreRF, you could do an Envelope Following analysis and plot the phase of the first harmonic of the carrier. I don't remember if the phase is directly accessible from the output form. If it is not, you might have to plot real and imaginary then use the waveform calculator to compute the phase.