Magnetic proximity effects are integral to manipulating spintronic(1,2), superconducting(3,4), excitonic(5) and topological phenomena(6-8) in heterostructures. These effects are highly sensitive to the interfacial electronic properties, such as electron wavefunction overlap and band alignment. The recent emergence of magnetic two-dimensional materials opens new possibilities for exploring proximity effects in van der Waals heterostructures(9-12). In particular, atomically thin CrI3 exhibits layered antiferromagnetism, in which adjacent ferromagnetic monolayers are antiferromagnetically coupled(9). Here we report a layer-resolved magnetic proximity effect in heterostructures formed by monolayer WSe2 and bi/trilayer CrI3. By controlling the individual layer magnetization in CrI3 with a magnetic field, we show that the spin-dependent charge transfer between WSe2 and CrI3 is dominated by the interfacial CrI3 layer, while the proximity exchange field is highly sensitive to the layered magnetic structure as a whole. In combination with reflective magnetic circular dichroism measurements, these properties allow the use of monolayer WSe2 as a spatially sensitive magnetic sensor to map out layered antiferromagnetic domain structures at zero magnetic field as well as antiferromagnetic/ferromagnetic domains at finite magnetic fields. Our work reveals a way to control proximity effects and probe interfacial magnetic order via van der Waals engineering(13). Controlling the individual layer magnetization in CrI3 enables the observation of a layer-resolved magnetic proximity effect in WSe2/CrI3 heterostructures.