You can find out where sendmail is, information about it including whether you have permission to run it, and similar information about /usr/lib/sendmail, with commands like these:

$ which sendmail
/usr/sbin/sendmail
$ ls -l /usr/sbin/sendmail
-rwxr-xr-x 1 root wheel 243824 Jan 13 18:14 /usr/sbin/sendmail
$ ls -l /usr/lib/sendmail
ls: cannot access '/usr/lib/sendmail': No such file or directory
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You can get similar information by writing a script that uses the file test operators.