We can see the pending rewards for the account over the week and tell if the author has already been well rewarded. This makes it hard for a nomination.
I never really understood this general rule. Especially when a lot of the top earners are making near $80 per post that are enforcing that rule to begin with. Admittedly over posts that aren't to the same degree of creativity or effort as those who make $10 consistently and considered "well rewarded".