According to the wikipedia page, a keystone species is:
A keystone species is a species that has a disproportionately large effect on its environment relative to its abundance.
Thus, you could work out the keystone species in an ecosystem by sampling population size and and what size 'effect' the removal of the species from the ecosystem would have. If you standardised the units of 'effect' for all species in the ecosytem, the keystone species would be those with the highest effect/population fraction. I don't know if there's a critical size of this fraction for a species to be determined 'keystone', but if there is it would allow you to explicitly define a keystone species. This would rule out the problem of interdependency as you mention.