A channeler can sense another channeler, under various conditions:

1. A female can sense another female, even if neither of them are holding the Source at the time. This is a skill which takes some time to learn. If one is really good, it is not necessary to see the other woman in order to know she is there. For example, one of the Tower AS senses Eg in the throne room in Cairhien, even though she was invisible. In addition, a female can tell when another is embracing the Source, by a great glow surrounding the channeler. Of course, female channelers can sense active female channeling, as well as the presence of other female channelers.

    

2. Females cannot naturally sense male channeling. It may be that they can do so by using the OP somehow, although Moggy's "technique" of [LOC: 8, The Storm Gathers, 178] was simply a trick.

    

3. Male channelers can sense female channeling, by a prickling feeling on the skin. There is a finite range on this. We can get some sense of the range from [TFOH: 15, What Can be Learned in Dreams, 213] where Moiraine is using the OP to eavesdrop on Rand and Asmo. Moiraine's tent is said to be "not far" from Rand's, let's guess around 20 meters. Note that the amount of channeling going on is probably very small; a bigger flow would probably increase the detection range.

    

4. Male channelers can sense male channeling, as well as other men holding the Source [LOC: 3, A Woman's Eyes, 92]. The sensing is not as obvious as when females sense other females. It is likely that it is more difficult for men to sense other men channeling at a distance. In [TFOH: 3, Pale Shadows, 75], Rand thinks, "Women who could channel saw a glow surrounding another woman who had embraced saidar and felt her channeling clearly, but he never saw anything around Asmodean, and felt little." This is with Asmo in the same room.

   However, there seems to be some confusion of exactly how far the range on male-male sensing is. Here are some examples:

   • [TFOH: 55, The Threads Burn, 664]: When Rand is running around trying to balefire the hell out of Rahvin, even though Rahvin is holding saidin, Rand can't feel where he is.
   • [TFOH: 55, The Threads Burn, 662]: Rand recalls Asmo describing trying to locate another man channeling as "like trying to find a lion in high grass." (That is, very difficult.)
   • [TFOH: 22, Birdcalls by Night, 287]: After a Draghkar attack on Rand's camp, Asmodean tells him that he had sensed the Draghkar, but had not done anything about it, since he thought Rand could deal with it, which he did. Rand replies: "'Well for you that you didn't,' Rand cut him off, sitting cross-legged in the dark. 'If I had felt you full of saidin out there tonight, I might have killed you.' "The other man's laugh was shaky. 'I thought of that, too.'" This little snippet of conversation seems fairly clear. Rand obviously felt capable of sensing Asmo from as far away as Rand was from Asmo during the fight, and Asmodean also thought it was possible.
   • [LOC: 42, The Black Tower, 545]: Rand says to Taim: "If I feel a man channel in Caemlyn...and don't think you can stay far enough from the Palace that I won't feel it and be safe." (NB: Rand may just be bluffing, here. Or it might just be something along the lines of "If I hear you use that sort of language in this house, young man, I'll wash your mouth out with soap!")
   • [TFOH: 44, The Lesser Sadness, 495]: Rand was able to sense lightning from Sammael (he thinks it is Sammael, at least) during the battle of Cairhien.

    

5. For both men and women, the ability to sense channeling seems to depend on the strength of the particular flow being channeled. For men, this is implied by Asmodean when he talks to Rand about blocking Aviendha's gateway to Seanchan: "I felt your weave - anybody within a mile could have felt it - I never saw anything like it - I didn't know that anyone but Demandred could block a gateway that was closing." [TFOH: 32, A Short Spear, 374]. For women, the dependence is implied by Elayne, Nynaeve, and Sareitha's detection of the Seanchan channeling in Ebou Dar at [TPOD: 5, The Breaking Storm, 129] - Elayne thinks, "Sareitha was not strong enough to sense saidar being wielded at that distance... She was not strong enough. Unless someone was using as much as they had on this hilltop." This quote shows that, for females at least, the ability to sense flows depends on both the strength of the flow and the strength of the woman doing the sensing. This may also be the case for men.