The normal answer in Holland is: "We have them seen by an objective medical doctor, not their own GP or specialist, and they decide". That seems fair enough, in a way, until you realize that:
(a) These "objective medical doctors" are usually from the medical dregs i.e. the worst doctors who are unable to find better employment
(b) These "objective medical doctors" are forced to declare a pre-determined percentage of people who say they are too ill to work as fit to work (pre-determined politically or bureaucratically)
(c) There is no realistic appeal against their decisions (court are overworked anyway; judges are not medical people; going to court requires money and health)
(d) These "objective medical doctors" are these days in Holland employed by private firms and private insurance companies whose interests they serve (and their interest is to pay out as little to ill people as possible)
(e) Most things related to medicine in Holland over the past 10 years have been massively turned over from the state to the health market, on the basis of the argument that "folks are then free to choose the best from genuinely competing firms" (as if selling cars and helping people medically are on a moral and economical par)
(f) There are now in Holland, and apparently also in England, and quite possibly elsewhere, lots of market forces in the form of commercial healthcare-firms who are paid from the taxes to do the bidding of the state.