Use a file manager to look in the following locations:
/data/app
/data/app-private
/system/app/
/sdcard/.android_secure
(shows .asec files, not .apks)
On Samsung phones:/sdcard/external_sd/.android_secure
You need to be rooted to view the first three.
Use command pm list package -f
(via adb shell
if you want to use ADB) to list packages with their respective locations. You can then read the files. This can be done without superuser permissions.