99 Intrepid consuming oil

My dad's got a 99 Intrepid with a 2.7 liter, and 67,000 miles. Before their trip from Wisconsin to Missouri, he checked the oil and noticed it was about a quart low. So he added a quart, but when they got to Missouri, saw it was a quart low again. No noticable oil leaks, and no visible signs of smoke, that he can see. what could it possibly be, and how could I check for this? Thanks.