Yoreh Deah Siman 18 Seif 17

יז טַבָּח שֶׁלֹּא הֶרְאָה סַכִּינוֹ לְחָכָם , הָיוּ מְנַדִּין אוֹתוֹ. וְהָאִדָּנָא נָהֲגוּ לְמַנּוֹת אֲנָשִׁים יְדוּעִים עַל הַשְּׁחִיטָה וְהַבְּדִיקָה, וְלָהֶם מָחֲלוּ חֲכָמִים כְּבוֹדָם, כִּי הֵם זְהִירִים וּזְרִיזִים; וְהַרְבֵּה צָרִיךְ יִשּׁוּב הַדַּעַת וְיִרְאַת שָׁמַיִם לִבְדִיקַת הַסַכִּין; הֲלֹא תִּרְאֶה כִּי יִבְדֹּק אָדָם פַּעֲמַיִם שָׁלֹשׁ וְלֹא יַרְגִּישׁ בִּפְגִימָה דַּקָּה, וְאַחַר כָּךְ יִמְצְאֶנָּה כִּי הֵכִין לִבּוֹ בָּאַחֲרוֹנָה; וּבְחִינַת חוּשׁ הַמִּשּׁוּשׁ כְּפִי כַּוָּנַת הַלֵּב

Seif 17

A butcher who doesn’t get his knife seen by a scholar, we place a ban on him.

Presently our practice is to appoint well known people to slaughter and inspect,

So the scholars gave up this honor to check by themselves and trust the shochtim, because they are careful and diligent.  There is need for a lot of clarity of mind and fear of heaven in checking the knife, If this is not apparent look that he checks it 3 times without feeling the damage for one minute, afterwards we find what he slaughtered recently, we test him to get a sense and feel in accordance with the intention of his heart.