For me - I get a split-second flavor flash that I can only describe as “dirty ashtray” in the back of my throat. When I get that - I know a cold will be arriving in a day.
Whatever I’m coming down with, my body lets me know I’m gonna be sick by sending me off to Dreamland. Since I was a baby, I deal with illness by sleeping. Sometimes, I can sleep enough to defend against a bug and not get sick. Otherwise, I just want to sleep through my illness. Scared my now-husband the first time he saw me sleep 14 hours+ without moving - he actually called my mom to reassure him that I wasn’t dying! (And, apparently, he kept checking to make sure I was breathing, since I wasn’t moving.)
TMI:
My balls get very sore - to the point that it’s uncomfortable to sit and only less so to lay down. 
Yeah, I can feel my sinuses. They don’t hurt or anything, but I can feel them. If I immediately start on the vitamin C/echinacea/zinc regimen, I can fend it off pretty successfully.
I usually find I start grinding/clenching my teeth really badly, and not just at night. When I realize mid-day that my jaw hurts from clenching my molars I usually find I feel the symptoms of a cold coming on a few hours or a day later.
I feel completely exhausted and run-down for a day or two, usually coupled with a sore throat and swollen-feeling ears. I usually try to knock it out with a massive dose of zinc and vitamin C when I first notice symptoms, which usually works, but not always.
For me it’s being unable to sleep. I just lie in bed awake. Nothing cycling in my mind or anything. Just awake for hours. The actual illness starts in 12-24 hours.
Nosebleeds.
About 2 days before the cold strikes, I get nosebleeds. Mild nosebleeds = not very sick. Heavy nosebleeds = very sick.
The first symptom I notice is a shift in my sense of smell–things start to smell sour (especially floral scents, for some reason) and sour odors become much more intense. By the time the primary symptoms (congestion, sore throat, and such) kick in, the scent of something like mustard can actually cause pain, as if it were burning my sinuses.
While laying in bed, and in a sort of half asleep half awake mode, I get this dream like scenario. Very hard to explain. It shifts from black to white and grows and shrinks. there are also spiky like protrusions coming from it. Cant explain it any better than that.
This happens exactly two days before I get sick. It has been happening since I was around 12-13, im 33 now. No matter what type of sickness I’ve had it always happens without fail.
I would love to hear an explanation of this. maybe im just nutso!
For me, it’s an ache where the hard and soft palates meet. Sometimes Vitamin C and Echinacea will ward it off, but usually it means 9 days of cold has started.
Also, I seem to do my very best singing about 2 days before the onset of a cold…
Huh? Really?