We can make mac to boot in verbose mode permanently (i.e, show all log messages during boot) by setting the "nvram" variable in terminal.
$ sudo nvram boot-args="-v"
This will set the mac to show verbose messages when every time mac os x boots.
To reset back again, run :
$ sudo nvram boot-args=
Note : If you want to see the verbose messages for momentarily then hold " Command+V " during the mac booting immediately after mac starts booting