Mainly through direct observation. Among my own circle of friends, it's surprising how neatly we (well, those of us who use either language) divide into those categories. But you can see the pattern in job ads, discussions on mailing lists and in the domain-specific libraries for each language too. As many people got into Python through NumPy as a MATLAB alternative as got into Ruby via Rails, I'll bet.

Like I say, this isn't a good thing or a bad thing, it's just interesting.