Most states have a statewide definition of limited English proficiency.
In New York, pupils with limited English proficiency are those pupils who by reason of foreign birth or ancestry, speak a language other than English, and for the 1990-91 school year or thereafter, score at or below the 40th percentile, or its equivalent, as determined by the Commissioner, on an English language assessment instrument approved by the Commissioner. Give it a try, and then check to see if you were even close.