Yoreh Deah Siman 24 Seif 18

יח נִמְצָא הַסִימָן הַשָּׁחוּט שָׁמוּט; אִם כְּשֶׁשָּׁחַט תָּפַשׂ הַסִימָנִים בְּיָדוֹ, אוֹ עוֹר בֵּית הַשְּׁחִיטָה מֵאַחֲרָיו, וְנִדְחַק הַסִימָן תַּחַת הָעוֹר, הֲרֵי זוֹ פְּסוּלָה; וְאִם לָאו, מֻתָּר עַל יְדֵי בְּדִיקָה, שֶׁיָּבִיא בְּהֵמָה וְיִשְׁחֹט הַסִימָן וְאַחַר כָּךְ יַעַקְרֶנּוּ, אִם דּוֹמוֹת שְׁתֵּי הַשְּׁחִיטוֹת זוֹ לָזוֹ, כְּשֵׁרָה; וְאִם הַשְּׁנִיָּה מַאֳדֶמֶת יוֹתֵר, פְּסוּלָה הָרִאשׁוֹנָה. וְעַכְשָׁו אֵין אָנוּ בְּקִיאִין בִּבְדִיקָה זוֹ, הִלְכָּךְ בְּכָל גַּוְנָא אָסוּר.

Seif 18

If a sign [either the windpipe or gullet] of the slaughter was found dislodged: if when he slaughtered he took hold of the signs in his hand or the skin from behind the place of slaughter and the signs were squeezed under the skin, this is not acceptable. If not, it is permitted after examination. If he brings cattle and he slaughters the sign and afterward it was dislodged, if it is the same in two of the slaughters,  comparing one to the other it is kosher. If the second is bloodier, the first one is not acceptable. We no longer have experts to check this and the custom in all of these cases is to rule them as not acceptable.