I have had frequent urination for 8 months had a urine culture and a urinalysis both negative. What other tests determine the cause of this problem?

More tests. Dynamic testing of the bladder, which help determine whether the bladder if emptying fully is another test you may need to have with your urologist. A pelvic ultrasound may also be helpful to assess for uterine fibroids, endometriosis, etc.
See below. Diabetes mellitus can cause frequency. Kidney disease can cause frequency. Diabetes insipitus can cause frequency. See your doctor for physical exam and lab work. Perhaps for a renal ultrasound to eval further. Prostatitis and prostatic enlargment might also cause frequency if you are male see your doc for phy exam and psa level.