In this paper we consider the extensions of description logics that were proposed to represent uncertain or vague knowledge, focusing on the fuzzy and possibilistic formalisms. We compare these two approaches and comment on their differences concentrating on the consistency issue of knowledge bases represented in these extended frameworks. We present a classification of existing algorithms and describe a new method for the possibilistic case that yields an inconsistency degree and not only a binary answer to the consistency question. The proposed algorithm is based on a direct extension of the tableau algorithm to the possibilistic case, for which we introduce appropriate clash and completion rule definitions.