see if an object is being seen?

how can i see if an object is being seen by the frustum? if its inside the frustum i mean, how is possible to perform this check? what are the basic involved to do so? i would like to learn the hard way by knowing first what i should do and later on if possible read some example code but that would be much to ask i guess it also woudlnt be the hard way would it? :stuck_out_tongue:

im just starting so dont be hard on me please
thank you!

Take a look here: