  Sarah: Well, I'm more than happy to tell you. I get used to this kind of thing, it being my job and all. Tok'ra (and Goa'uld) have naquada in their blood. Now, when other naquada is in the vacinity, the molecules polarize, positive end toward the source, sending a signal to the brain that there is naquada nearby in a particular direction. Since other Tok'ra, Goa'uld, and Jaffa, as well as those who were once one of the above have the naquada in them, they can be sensed by Tok'ra like myself and Khala.
Naquada was also a particular period in Ancient Egyptian history here on Earth. It's probably the era just before Ra's foothold in Egypt was overthrown and the Stargate buried. Vinnet: This was all derived from Major Carter's research on the subject; we Tok'ra have neither the time nor interest for such matters. For us, it is not so much why we can sense each other as it is that we can. 
