For at least a little while you best bet will be a infrared camera to observe them from a distance. While they may work hard to mimic the characteristics you can see, they may not have tried to mimic thermal emmittance patterns yet.  Simularly, their density is likely to have changed as silicon has heavier nuclei than carbon.  One could place a scale in thoroughfares or require them to step on it before meeting with you. As I do not know what molecules they are made out of, however, I cannot predict how these would change.  I can only know that they likely would have.

An IR spectrometer would work well if their skin is still made of a silicone matrix. But that would only help when the start finding a way to mimic your blood. As you need a reliable sample which means you need to take it.