What is the test for the doctrine of "head of power" in Canada?

The basic theory is that the heads of power enumerated in section 91 are not only grants of power to the federal government but, being exclusive powers, are also denied to the provinces. Some say illogically, the doctrine does not protect provincial powers from federal legislation. ( Friends of the Oldman River Society v. Canada 1992 CanLII 110 (SCC), [1992] 1 S.C.R. 3, at paragraph 99).

Hogg has criticized the rationale for this doctrine (in Chapter 15.8(e)). In particular, he has trouble reconciling this doctrine with the necessarily-incidental doctrine (which he calls the “pith and substance” doctrine), which analysis stipulates that a valid provincial law may incidentally affect a federal matter. He observes that the latter analysis is applied more frequently than the former. He reluctantly acknowledges the “effort” in Bell Canada v. Quebec 1988 CanLII 81 (SCC), [1988] 1 S.C.R. 749 (“Bell 1988") to define the boundary between the two doctrines.