Yoreh Deah Siman 23 Seif 6

 

ו שָׁחַט עוֹף וְשָׁהָה בּוֹ, וְאֵינוֹ יוֹדֵעַ אִם נִקַּב הַוֵּשֶׁט, חוֹזֵר וְשׁוֹחֵט הַקָּנֶה לְבַדּוֹ בְּמָקוֹם אַחֵר, וּמַנִּיחוֹ עַד שֶׁיָּמוּת, וְהוֹפֵךְ הַוֵּשֶׁט וּבוֹדְקוֹ מִבִּפְנִים; אִם לֹא נִמְצָא בּוֹ טִפַּת דָּם, בְּיָדוּעַ שֶׁלֹּא נִקַּב וּכְשֵׁרָה.

 

Seif 6

If one slaughters a fowl and pauses without knowing if he perforated the esophagus, he should return and slaughter the windpipe in another place and wait until it dies. He then turns the esophagus out and checks it [the cuts] against each other. If spots of blood are not found, it is known that there are no perforations and it [the slaughter] is acceptable.