How can I fix my Mansfield No. 160 Toilet?

My Mansfield No. 160 toilet sounds like it is filling at night time - long after it has been used. I do not know if it is the fill valve or the flush value that is the problem, but it seems that if it is filling. This occurs just briefly, but I am concerned that it is costing me in water. Any suggestions? Do I rebuild the entire insides or can it be determined what the specific problem is?