When choosing a wife among pious candidates, how does one tell which ones would make a good mother of children?
The best way to answer this is with a number of questions for to ask yourself and her. If you can answer these and are comfortable with the reality of these answers, then consider marriage, but you are not going to line up a bunch of pious candidates and just pick one. Marriage isn't a fairy tale, its a life long journey of struggle, learning, and commitment. There are happy times, but it sometimes takes the hard times to appreciate the good ones.
How many siblings does she have?
How is her relationship to them?
How is her relationship to her parents?
How is her parents marriage?
When you marry her, you marry the family. And even though a 20 something might have it "all figured out," they default back to what they know and understand...How they themselves were raised.
How is she nieces and nephews, or other people's children?
Is the way she was raised similar to your own upbringing?
How does she react when frustrated or angry?
How does she react when hurt?
How does she react when you are frustrated, hurt, or angry?
What are her views on Catholicism? I mean does she believe in the same traditional Catholic sect as you? The numbers of sects, and cults of personalities are growing, if/when the one you are following takes a left turn, can you talk it out with her? Will she raise your children in agreement to what your both decided?
How educated is she?
If you are injured/sick/laid off, could she find work to help support the family?
Does your parents/family approve of her? Sometimes parents may know a thing or two and be able to read a situation better than you can.