Résumé
J. Berstel and P. Séébold have proved that an acyclic morphism f is Sturmian iff the word f(baabaababaabab) is balanced. More precisely, they have given a set of test-words for Sturmian morphisms. Here, we characterize all such test-words. In particular, we show the optimality of the previous result: there is no test-word of length less or equal to 13, and any test-word has a subword in To do this, we describe an efficient algorithm to determine if a finite word is balanced, and we give a short proof of the fact that any finite balanced word is a prefix of an infinite Sturmian word. Finally, we show that the test-words for Sturmian morphisms are exactly the test-words for morphisms preserving finite balanced words.