Read that a person can have a normal cmp, cbc, and urinalysis and still have kidney damage. so then how would a person ever know? do they just get kidney problems or worse suddenly ?

One would not usually have a normal CMP, CBC, and urinalysis and have abnormal kidney function. Usually one of those may reveal a problem with function. Although there are kidney structural issues which these lab tests would be normal. Like cysts, stones. Very early kidney disease may not reveal lab abnormalities. We look for kidney disease if there are symptoms or concerns from an evaluation.

Normal CMP and urinalysis could not rule out CKD because serum creatinine which we currently use as a marker for kidney function is not sensitive enough. Serum creatinine levels over the years is a better index. Serum creatinine is only out of range when you lost more than 50% of your kidney function. A kidney donor under 50 years old will have normal serum creatinine three months later.