<p>The execvp() function executes the program that is given as an argument. It checks the $PATH variable to find the program. I'm writing something in which I would like to check to see if several programs exist before calling any exec() functions. What's the best way to do this?</p>

You can use getenv to get the PATH environment variable and then search through it.
http://www.opengroup.org/onlinepubs/000095399/functions/getenv.html
You can then use fopen to check for the existence of the specific binary names.
You can also do something like system("which App"). which searches $PATH for you.
http://en.wikipedia.org/wiki/System_(C_standard_library)
http://en.wikipedia.org/wiki/Which_(Unix)