Page's misuse of Haldane's Dilemma

I've asked this question elsewhere before, and not got much of
an answer, but you seem much more knowledgeable so I'll try you.How do we know that there are few beneficial mutations, or that
deliterious ones outweigh beneficial ?A 'bad' mutation will have an effect which limits the mutant in
some way, perhaps even killing them outright.A 'good' mutation would, presumably, not impede the individual
and so go unnoticed.How do we detect mutations and calculate the 'good':'bad'
ratio ?