<div dir="ltr">Use \show<br><br>e.g<br><br>\show\TeX<br><br>Gives:<br><br>> \TeX=macro:<br>->T\kern -.1667em\lower .5ex\hbox {E}\kern -.125emX.<br><br>which means \TeX is defined and it is a macro not a primitive.<br>
<br>\show\pdfoutput<br><br>gives:<br><br>> \pdfoutput=\pdfoutput.<br><br>which means \pdfoutput exist and it is a primitive.<br><br>Best wishes,<br>Vafa Khalighi<br></div>