Q. I’m using CentOS on 64 bit Linux. How do I find out rpm package architecture before installation such as i386 or x86_64 bit?

A. Almost each rpm package has i386 or x86_64 added to file extension. However some time you may see filename such as file-version.rpm. You can list all installed rpm packages with rpm -qa command but it will not display architecture. You can run following command to display rpm package architecture:
rpm -qa --qf "%{n}-%{arch}\n"
Ouput:

filesystem-x86_64
comps-extras-noarch
gnome-mime-data-x86_64
glibc-x86_64
atk-x86_64
libICE-x86_64
db4-x86_64
elfutils-libelf-x86_64
ncurses-x86_64
libsepol-x86_64
libcap-x86_64
sqlite-x86_64
pcre-x86_64
gdbm-x86_64
libsepol-i386
libXdmcp-x86_64
db4-i386
iptables-x86_64
libXrender-x86_64
libXi-x86_64
xorg-x11-font-utils-x86_64
slang-x86_64
libXpm-x86_64
startup-notification-x86_64
gzip-x86_64
libIDL-x86_64
libXau-i386
wireless-tools-x86_64
libvorbis-x86_64

Another option is to use following command to display detailed information about package:
rpm -qi {package-name}
If you wish to display package version along with architecture, enter:
rpm -qa --qf "%{n}-%{v}-%{r}.%{arch}\n" | less
Run rpm command to print a list of all of the tags it knows about when it is invoked with the –querytags argument.
rpm --querytags | less