how do you know if its the altinater or not?

i have a 2003 ford taurus ses 3.0 doc duretec and its is starting to loose power.and draining the battery we had the battery tested and it is fine.so how do we know if it is the alt.or not.and also how in the world do you get it out it looks really complicated.is there a seceret or easy way. thank you.