Like you said, people are different, and no two cases of Lupus are the same. However, I've noticed in my time here that there are some definite commonalities many of us share.
For me, the most frequent and sure fire sign that I am going into a flare, is mouth sores, and neck pain. I get sores on the roof of my mouth mostly, and they start tingling and burning a bit before they totally break out. The neck pain I get is a line of pain up from my right shoulder into the back of my head. Turning my head hurts, and it makes sleeping difficult.
Another sign that I get maybe 50% of the time, is a burning itch that seems to come from the inside, under the skin rather than topical like a bug bite or sunburn feels. I get it mostly on the fronts of my thighs. It's a maddening kind of itch, like hot needles from the inside.
To try to head off the flare before it gets really bad, I load up on NSAIDS-mostly Naproxen Sodium, which is sold under the brand name Aleve. It can be hard on the stomach though, so I always take this med with food, like bread or crackers. I use a Steroidal Cream for the itching on the legs, and I use topical anesthetic for the mouth sores. The mouth sores are really the worst. The anesthetic only numbs the area for a short time, and the stuff tastes awful. Eating a slice of toast feels like munching on broken glass. OUCH! I end up eating soft stuff, like yogurt, and ice cream (not a bad thing).
I also try to avoid the sun completely if possible when a flare starts, more so than I usually do. Of course, plenty of rest and avoiding stress will make a big difference as well.
Hope this helps,
ETA-I forgot to mention my other definite sign of an imminent flare-Cognitive Dysfunction, AKA Brainfog. I get a distinct effect in the form of overall forgetfulness, a decrease in my ability to spell and formulate a sentence, and an inability to communicate face to face or on the phone-I just draw a total blank.
It's rather ironic that I forgot to mention how Brainfog makes me forgetful.