Discern false modesty from the genuine (one)?
I see that people have answered in the comments (naughty!). I'll answer the way I was going to anyway.
Answer
She was not good at discerning false modesty from genuine. (genuine is an adjective so does not take an article)
or
She was not good at discerning false modesty from the genuine sort. (sort is a noun and so takes an article)