I'm not familiar with Baum's Oz universe enough to know for sure, but it seems to me that in most cases, true experts in any field can spot a fraud. So I'm simply wondering if there is anything mentioning that the witches (any of them) know that the Wizard was a fraud, but chose not to expose him for some reason, or were they truly fooled?
In the opening chapter of "Glinda of Oz", the fourteenth (and last written by Baum himself) of the series, the audience is introduced to "Glinda's Great Book of Records". The book is described thus:
on the pages of which are constantly being printed a record of every event that happens in any part of the world, at exactly the moment it happens. And the records are always truthful, although sometimes they do not give as many details as one could wish.
So it seems entirely likely that Glinda, using the Book of Records, would know the origins of the Wizard and his actual magic abilities (none).
On the other hand, it is a virtual certainty that none of the wicked witches knew. If they had known, they would certainly have attempted to kill him (they were, after all, wicked). Their ability to succeed in killing him would not necessarily have been a sure thing. While the Wizard had no magic abilities, he exemplified some of the better qualities of stage magicians including creativity, resourcefulness, dedication, attention to detail plus a better than average understanding of people.
There is an interesting thing about the Wizard.
In the fourth book of the series the Wizard returns to Oz and eventually becomes an apprentice of Glinda's. By the seventh book he was capable of performing significant (real) magic of his own.
In the original book, the witches definitely don't know about the true nature of Oz: the wicked witch of the East feared Oz enough to use one of the precious uses (out of 3) of the winged monkeys against him.
Edit: It is very likely that the good witch of the North also didn't know who Oz really is, because she suggested Dorothy to travel and seek his help.