This question is for my thesis which exploring the internet infrastructure, internet of things and aspects of object-oriented ontology in building device-device relationships.
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You can look at Information Theory. There is a Shannon channel between to IoT devices (say machines, communicating with an M2M protocol). Then the information exchange can be monitored and measured using entropy, mutual information (Kullback-Leibler). So the symbol probability at device 1, say P1 is monitored, and the same goes for device 2, with P2, then look at the D(P1||P2) (see wikipedia "Kullback Leibler information").
This is a very fundamental way.
You can have a more pedestrian approach, and say that device 1 produces x1(t), and device 2 produces X2(t), look at the correlation matrix m(i,j)= E(x1(i).x2(j)), and diagonalise it (it is a Gram matrix, see wikipedia for definition and property).
Then you identify commmonality and difference between 1 and 2...