Two key elements contribute to a text’s readability: word length and sentence length.
You can input these elements into different formulas, including the Flesch-Kincaid scale and the Fry graph. They will then tell you what level of education your reader will need to understand most of your text at first pass. The lower the better, especially for public readers.
Just don’t rely solely on readability measures to tell you whether your text is ‘in plain English’ or not. They represent just 1 in a suite of factors that contribute to clear writing.