Separating normal training soreness from a sign to back off without diagnosing an injury

Some soreness the day after a new workout seems normal. But I don't want to turn ‘no pain, no gain’ into a reason to ignore warning signs. I'm not asking for an injury diagnosis—just a sensible training decision rule. How do you distinguish ordinary, symmetrical muscle soreness that eases with movement from pain that warrants stopping an exercise and getting professional advice? I tend to be too stubborn about finishing the plan.