From an ab-initio simulation point of view, it would be the same calculation.
Hand-waving we would call reaction with a low energy gain for physisorption and those with a high energy gain for chemisorption.
Another way to quantify it would be to look at the molecular levels of the molecule alone versus the molecule on the surface.
According to Wikipedia:
Physisorption, also called physical adsorption, is a process in which the electronic structure of the atom or molecule is barely perturbed upon adsorption.
Therefore if you compare the molecular spectrum of both cases, and they are similar, you could classify it as physisorption. If they are significantly changed, then you could classify it as chemisorption
