The overlap between nonorthogonal wave functions of resonance (Siegert) states is analyzed. Their asymptotic behavior, which controls the extent of nonorthogonality, is shown to be directly related to the residues of the energy S matrix. This allows one to estimate overlap integrals using solely positions and widths of resonance states, without the explicit calculation of the wave functions. This assessment, as applied to NO2, suggests that resonances in the molecule are only slightly nonorthogonal and decay independently of one another.