If Strategy is set to Determinantal, isVirtual will check whether the given chain complex is a virtual resolution by checking the depth of the saturation of the ideals of maximal rank from the boundary maps. See Theorem 1.3 of [Loper, arXiv:1904.05994].
The source of this document is in /build/reproducible-path/macaulay2-1.25.06+ds/M2/Macaulay2/packages/VirtualResolutions/doc.m2:143:0.