דברים כב ,ב: וְאִם- לֶא ֹו תְפַּסֲאַּו ֹו תְעַּדְי אֹלְו
ךיֶלֵא
ךיִחָא בֹורָק אֹל-
ךְ מִע הָיָהְו
ךֶתיֵ ב
ךֹו ת
ךיִחָא ׁשֹרְ ד דַּע
אֹתוֹ וַּהֲשׁ ֵבֹתוֹ לוֹ:
Baal HaTurim cites the Gemara in Bava Metzia that by translating "oso" as 'its sign' (of identification), one can switch the phrase to the finder. In that case, what the Pasuk is now saying is that someone who finds a lost object may not return it to anybody before cross-examining him. How? One must demand that the “owner” provides him with the object's double identification marks.
The term “double identification marks” is unclear. The Maggid Mishneh distinguishes between three classes of simanim (identification marks). The best are called “extremely clear simanim” such as a hole next to a particular letter. Torah law requires that such a found article is returned to the claimant on the basis of such a siman. This may be the type of siman described as doubled, since it clearest of the clearest.
The next class is “worthy simanim” such as the dimensions of an object. In this case the article also must be returned (but maybe only according to the Rabbis).
Finally the weakest type of siman such as “it’s long” or “it’s heavy” is not an acceptable proof of ownership.