There are many telltale signs as to why a particular quotation should raise suspicions about its authenticity.  One of the clearest cut ways, however, to decipher the legitimacy of a quotation, is to know the supposed author well.  Thus, when we have a quotation from Plato that goes like this,

A hero is born among a hundred, A wise man is found among a thousand, but an accomplished one might not be found even among a hundred thousand men.

there is every reason to be suspicious.  That is, it is unlikely that Plato would say that there is any person more rare than a wise man (and thus more valuable).

Verdict: This quotation is not by Plato.

It is from a translation of The Staff of Wisdom commonly (but not without dispute) attributed to Nagarjuna, an influential Buddhist philosopher, which can be found in the book Elegant Sayings (Dharma Publishing, 1977, p.31).