Occasionally in the synagogue the one reading the Torah will notice an improperly-written letter in the Torah scroll, and a child will be called upon to determine whether or not the letter in question was written correctly. Under what circumstances can this system be used to determine the validity of a Sefer Torah?
The Shulchan Aruch writes (32:16) that a child is called upon only in situations of a possible problem with a "one-legged" letter, where the "leg" was not drawn to the proper length. For example, a final "Nun" that was not drawn properly may resemble a letter "Vav"; a "Vav" drawn too short will resemble a "Yod." By the same token, a final "Chof" whose "leg" does not extend as it should looks like a "Dalet." In all these situations, a child is called upon and asked to identify the letter in question. If he identifies it as the intended letter, then the congregation may rely on his determination and use the Sefer Torah.
This method may not be used in situations where the letter lacks the proper "Tzurat Ha'ot," meaning, the basic structure and form of the letter is deficient. For example, the letter "Alef" is drawn with a diagonal letter "Vav" with a "Yod" protruding from the lower left and another from the upper right. If one of the "Yod's" does not reach the diagonal "Vav," then even if a child identifies the letter immediately as an "Alef," the Sefer Torah may not be used. In this case, the letter lacks the basic configuration required for that letter, and the Torah scroll is therefore invalid. This case differs from the situations described earlier, where the letter is basically drawn properly, and the question involves only the length of its "leg."
Summary: In situations where the "leg" of a letter in a Torah scroll is not drawn to the proper length and thus resembles another letter, such as a final "Nun" that resembles a "Vav," a child is called upon to identify the letter in question. If he identifies the letter in question as the intended letter, then the Sefer Torah is valid. In other situations, however, where a letter lacks the basic configuration of the intended letter, the Sefer Torah may not be used, and the congregation may not rely on the child's identification of the letter.
See Halacha Berura ibid.
