How would we know if modern people were of lower 'fitness' (due to mutation accumulation) than people in the past?

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Fitness (the probability of reproductive success) is something which has so many aspects that - except with the obvious case of extreme low fitness leading to rapid death - relative fitness can only really be evaluated in practice, in situ, in an environment.

So to compare the fitness of adults in 1800 with adults in 2014, the best method would be put people of 2014 into a functional equivalent of the conditions of 1800 - with the same kind of diet, diseases, housing and working conditions; the same lack of medicine, lack of hygiene, lack of cocooning provision; lack of contraception and safe abortion; lack of formal education and the mass media, and the rest of it - and see how well modern people reproduce under these conditions.

It would not be the same! since eight or nine generations have passed with major changes in selection pressure - so moderns would either be significantly better, or they would be significantly worse at reproducing under 1800 conditions.

If moderns are worse at producing viable offspring under 1800 conditions, moderns could reasonably be considered to have declined in fitness - unless it was asserted that moderns have evolved new adaptations in eight generations; which is implausible.

(Implausible because the process of losing adaptations is much faster in generational terms than the incremental building of adaptations - and because the strength and nature of selection pressures would appear to have been changing very rapidly over the past 200 years.)

Alternatively, genetic samples of modern ties could be compared with those of 1800, and compared in terms of the proportion of what seems to be dysfunctional, cumulative mutational damage.

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