It is possible to have the defendant's ability to read English tested, and determine the grade level at which s/he reads and understands English. The language used for many of the state advisal statutes, e.g., California Penal Code 1016.5, warning is quite difficult, and it takes a reading level of at least 12th grade to understand 70% of the warning.
Microsoft WORD has the ability to compute the Flesch-Kincaid Grade Level score, which gives the U.S. grade level required to understand a given text, under Tools, Spelling and Grammar, Options, Grammar, Show Readability Statistics. When this operation was performed on the Penal Code 1016.5 text, it produced a readability score of 12th grade.
For a description of the process of challenging a defendant's competency to understand and waive Miranda rights, and the analogous question of challenging ability to understand the immigration waiver, see I. Bruce Frumkin & Alfredo Garcia, Psychological Evaluations and the Competency to Waive Miranda Rights, THE CHAMPION 12 (Nov. 2003); S. Kassin & G. Gudjonsson, The Psychology of Confessions: A Review of the Literature and Issues, 5 PSYCHOLOGICAL SCIENCE IN THE PUBLIC INTEREST (November 2004). If the defendant's reading grade level is below the readability score required to understand the warning, it is possible to argue that the defendant did not understand the warning, and the conviction must therefore be vacated. See, e.g., People v. Ramirez (1999) 71 Cal.App.4th 519, 522 (Penal Code 1016.5 advice can be given by means of a form, provided: "The judge need only determine whether defendant had read and understood the contents of the form, and had discussed them with his attorney."), quoting In re Ibarra (1983) 34 Cal.3d 277, 285-286.