For the most part, you are not going to find married women’s maiden names in the Canadian census returns (nor in the English or US census returns either, for that matter). Census enumerators were neither required nor expected to record married women’s maiden names; not surprisingly, most of them did not do so.
But occasionally (very occasionally indeed), you will find a census enumeration where the enumerator did record the maiden names of married women. A useful quirk, from the perspective of the genealogist, and a bit of a research bonus if you’re lucky enough to find such an emumeration.
From the 1861 enumeration of the Township of Algona in Renfrew County, here, for example, is the household of Cornelius Harrington, who is listed along with his wife Margaret Ryan (here spelled “Reyan”), his sister-in-law Johanna Ryan, and his children Michael and Bridget:
That “U C” — for the birthplace of Michael and Bridget Harrington — stands for Upper Canada. The “R C” of course stands for Roman Catholic.
My guess is that this census enumerator was Scottish (and I suppose I could look this up, but for now I’ll leave it as a guess). In Scotland, it was customary to call a married woman by her maiden name even after marriage. To be sure, most Scottish or Scottish-Canadian enumerators of the Canadian census followed the English practice (which was what they were instructed to do, after all) of listing married women by their husband’s surnames (which was a married woman’s legal surname, of course). But in the only other example I have seen of an enumerator of the Canadian census listing married women by maiden names, that enumerator was Scottish.