Question: I've often heard that the obesity of women in baroque art signs that such women were the ideal of beauty. How could we know that? And how could we know it wasn't just "cultural rationalization" of a standard physical appearance of higher classes?

Answer: 
Art does not exist in a vacuum, but is rather only one part of the historical record. Just as people comment on our modern standard of beauty today, so does early modern writers on theirs. Fortunately, Baroque art dates from a recent enough period that the historical record is extensive.

For example, a 17th century commentary on a Van Dyck portrait of a heavier women states:

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> William Sanderson in the treatise *Graphice*, published in 1658, noted that a beautiful woman was to have "a noble neck, round rising, full and fat . . . brawny arm of good flesh. Such a lady possesses a goodly plump fat."
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> Wind, Barry. *A Foul and Pestilent Congregation: Images of Freaks in Baroque Art.* Routledge, 2018.
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Beyond artistic contexts, we also find examples of beauty being explicitly attributed to body fat:

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> The use of fat is . . . It fills up the empty spaces between the Muscles, Vessels, and Skin, and consequently renders the Body smooth, white, soft, fair, and beautiful . . . Persons in a Consumption and decrepit old women are deformed for want of Fat.
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> Bartholin, Thomas, Caspar Bartholin, and Johannes Walaeus. *Bartholinus Anatomy: Made from the Precepts of His Father, and from the Observations of All Modern Anatomists.* John Streater.
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Historians, on the basis of information like this, therefore argue that people of the period - at least in some countries - considered it desirable for women to have a bit of plump. 

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As for "rationalisation" - of course, you can definitely make the case that they only considered chubbiness beautiful because of the association with wealth. However, does that necessarily invalidate their standard of beauty? I believe this delves into a philosophical realm over how beauty should be defined, which is beyond the scope of history.
A lot of the paintings were commissioned as portraits, why would people pay for themselves to be depicted in an ugly way? 

Wealth nowadays is associated with a slim, tanned, and shaped body because those are traits of people who have enough free time, and money to achieve it. In that period, it would be the reverse, being more on the fat side would require wealth, and more refined foods which were more expensive, while majority of commoners would be slim, tanned (for working in the sun) and toned due to hard work. 

It makes sense based on those, that it must have been acceptable to be fat back then, even desirable.