These binary files are mostly located in the below 4 locations
# which "command_name"
# which cp
alias cp='cp -i'
# which useradd
In case you get the below error# which samba
/usr/bin/which: no samba in (/usr/lib/qt-3.3/bin:/usr/local/sbin
It means your PATH variable does not contains the path for samba command. In that case which will not help you find the location of the command.
So, you can use the below command for the same purpose
# whereis command_name
# whereis useradd
useradd: /usr/sbin/useradd /usr/share/man/man8/useradd.8.gz