The last two books are more concentrated in time than the previous books, but since the first book, there has been a narrowing of the time frame. Simply because it's a talent that appears very rarely among men. The wise ones are doubtful that there actually can be a male dreamwalker. One of the themes of the book is that no one knows everything there is to know. Another is that just because you believe something to be true, doesn't mean that it is true.
Someone can judge a current strength.
This differes between men and women. A woman that can channel can very accurate judgement of another woman's strength whether she is channeling or not if she is standing close enough. Among Aes Sedai at least, knowledge of potential strength, especially if it is thought to be a great strength, becomes very wide spread. Among men the circumstances are different. A man who can channel cannot judge the strength of another unless the other is channeling the One Power of holding the One Power, and even then all he can judge is how much of the One Power the other man is holding.