Testing is increasingly recognized as a vital part of language revitalization. I demonstrate here that assessment of linguistic knowledge should also be part of the planning process that precedes the creation of a revitalization program. I take as an example Jejueo, the language of Korea's Jeju Island. Whereas previously published work contradicted UNESCO's conclusion that the language is critically endangered, a test that I designed to elicit basic vocabulary and verbal patterns from 224 participants (from elementary school students to senior citizens) revealed otherwise. Alarming deficits in basic knowledge of the language were uncovered that both confirmed UNESCO's classification of the language and identified the particular areas in which remediation is required.