You need to know what the name of the executable is to be able to run the program and where the executable is installed if it is installed somewhere that isn't in your path. Under Linux executables for all users are usually placed in /bin, /usr/bin, /usr/local/bin or ./usr/X11R6/bin. Executables that usually require root access to run are stored in /sbin and /usr/sbin.
When you install using an rpm you can find out what files are installed and to where. The format is: rpm -ql <packagename>
Example if you didnt know what the executable was for gedit:
$ rpm -ql gedit
<output snipped I just wanted to give you an idea>
Look at the very first line outputed: /usr/bin/gedit
That shows gedit is the name of the executable. Note: some rpms have more then one executable program so it may not be that easy al the time.
For software you compile and install from tar.gz files the best way would be to look at the docs. They will more then likely tell you how to run the program. If you want a tutorial on compiling and installing from sources look here.