  We propose a protocol to certify the presence of entanglement in artificial
on-surface atomic and molecular spin arrays using electron spin resonance
carried by scanning tunnel microscopes (ESR-STM). We first generalize the
theorem that relates global spin susceptibility as an entanglement witness to
the case of anisotropic Zeeman interactions, relevant for surfaces. We then
propose a method to measure the spin susceptibilities of surface-spin arrays
combining ESR-STM with atomic manipulation. Our calculations show that
entanglement can be certified in antiferromagnetically coupled spin dimers and
trimers with state of the art ESR-STM magnetometry.
