This may come as a shock to you, but Moms do not develop eyes in the back of their head when they are pregnant, nor do they get them surgically implanted post partum. I have recently come to discover the little secret to how my Mom "knew" when we did or did not do certain things.
Sound. Too much noise. No noise. Strange noises. Noises coming from strange places. They all can set off my Mommy Warning System.
Examples:
Sound: TV suddenly becomes muted, flip of the switch in the bathroom followed by a flush and then a little boy rushing back to unpause the TV.
Mommy: "Did you forget something?"
Boy: pause. "oh! wash hands!" (turns around and returns to the bathroom)
Sound: water running in the sink for a few minutes
Mommy: "What are you doing in there? washing your hair?"
Sight: little boy with liquid hand soap rubbed in his hair sticking his head under the faucet.
Sound: thud thud up the stairs, thud thud to his room, silence, THUD, thud thud back down the hallway and downstairs singing "and there who's clues? Blue's Clues."
Mommy thought process: "ugh! what is that going to sound like 10 years from now when the size 11 boys feet are size 11 mens? What is he doing up there anyway? oh, getting Blue from the bed and jumping off and here he comes back again."
My Mom also had a pretty good BS Detector. With me and my brother she probably needed it.
At this point J does not know how to lie per se. I have heard autistic children don't know how to intentionally lie. But, he does say things that are not true probably because he just gets in the habit of mindlessly answering yes or parroting back what some one else has said. When he does, his voice will take on a very distinctive monotone quality. The BS Detector beeps and I make sure he is paying attention then ask him the same question sometimes several ways to get the straight answer.
Example (sorry, a bit TMI):
Mommy: "Did you go poop at school today?"
J: "yes"
M: "With Ms Rebecca or Ms Michelle?"
J: "With Ms. Michelle" in the robotic monotone voice
M: "Look me in the eyes, please. Did you go poop when you were with Ms Michelle?"
J: "Not yet."
M: "How about with Ms. Rebecca or Ms Dana?"
J: "No"
M: "So you haven't gone poop today?"
J: "Not yet"
M: "Let's go try."
J: "Not yet"
M: "sigh" bang bang bang (mentally banging MY head on the wall)
My husband is often amazed by my ability to be getting dinner ready in the kitchen and to know J is around the corner playing parade with his train ornament and his wooden nativity set.

How did I know? I became suspicious the first time I heard the noise and checked on him and my brain was able to remember it.
Too bad I can never remember where I leave my keys. But that's another topic.
motherhoodismagic.com says
I've become a pro at differentiating noises that my children make! I also have a cabinet in the dining room with glass doors and if the doors are open to a certain angle, I can see what the kids are doing in the living room. It totally freaks them out when I yell from the kitchen for them to stop sitting on the coffee table. And I hear ya about the keys!
Mary~Momathon says
I heard my six year old say to my four year old, 'How did she know?" Oh yes, Moms know!