You can try looking for the telltale white deposits left by evaporating anti-freeze on the engine, hoses, radiator ect. I had a similar problem on a 98 Neon - topped it off every month or so for almost a year then one of the radiator tubes let go in a big way and problem found. Good thing it was just outside my shop for a tire check... it got tire pressure checked and a new radiator. No leak now. The bad place was hard to see with the radiator installed. Next time I'll try an inspection mirror.