The synergistic effect is in general based on the perspective usage of your composite which depends on the filler you have used. What type of filler is yours ? What for you have used your proposed filler in your composite ?
Ok. "Synergistic effects" implies your expectation that the combination of polymer and filler is better than the mere sum of the two. In this case, I'd prepare a systematic set of samples with increasing filler content and check the properties (thermal stability, thermal conductivity?). Then plot the difference to the reference vs content. If you get a straight line the effect appears to be additive, if you get a deviation from linearity in the expected direction, there might be synergistic effects. If you have an effect, the next question would be, whether you want/need to find out, where the effect comes from.