You know another thought occurred to me. According to Lucas, in the prequels the force is generally about midiclorians. And these midiclorians allow you to tap into the force like some kind of computer access. The more midiclorians you have the better access. Qui Gon didn't know (but had a feeling) that Anakin was a force user. He needed to take blood and make a positive match, therefore Lucas has established in the Prequels that a force user doesn't effect a "field", if so odds are Qui Gon would have detected Anakin miles away if Anakin isn't able to control the force. Thus Lucas establishes the only sure fire way to know is through a blood sample according to the prequels.
oh oh!
remember in the end of Star Wars, Luke is flying down the trench with Vadar on his tail. Vadar says rather causally, "The force is strong with this one?" Okay... how does he *know* that. This completely and 100% contradicts The Phantom Menace and it sets the tone that force users can sense each other through a "disturbance". Vadar had no familiarity with Luke and even moreso, Luke really didn't say anything as well when it came to sensing Vadar, probably because he doesn't know what to look for. Kinda like a master in meditation and a novice. The master says, "feel that?" and the novice goes, "feel what?".
I know I'm nitpicking, and really what difference does it make? It's just a movie, but I think this down right answers the questions with how the force works. The force is an energy field as Yoda describes, where people learn to tap into and effect that field. the universe acts upon it in a natural way but people if attuned properly can act upon it in an unnatural way and manipulate the field. How they were able to manipulate it I guess could have been explained by midiclorians but I like the idea of it being a simple field in a galaxy that is as inherent as air is to our world. But according to the end of Star Wars, a force user without familiarity or knowing the other is a force can sense a disturbance in the force to a specific point and even know what or who is effecting it. Thus, Palpatine as a Sith is a plot hole big enough for a Mack truck to drive through.