Looking around my area, I cannot differentiate from the ones who have been here a while and those who just arrived. I do see many working very hard, and it is often dirty, nasty work for which does not need much literacy. Also, I see Hispanics who have been here for a while and speak some English with crews of Hispanics who could easily be close to illiterate. I have lived, studied and traveled in Mexico at various times and places for 7 months and traveled by bus and train, and traveled as far as Nicaragua by bus or car. I can more or less tell what kind of background the workers I see probably have based on size and clothing. The poor are often smaller.
I know enough Spanish that when I ask a food worker what the Spanish word is for “pretzel” and she responds “cookie”, then she has a pretty basic vocabulary - and she has been here for years.
Interesting about the size; besides the obvious (nutrition), I wonder if it is also a reflection of less (or no) European ancestry. I read that North Koreans that flee into Red China are easy to catch because not only do they lack the Chinese language, but they are short (definitely from malnutrition) - they can be up to 6” shorter than South Koreans.