1998 Cadillac EL Dorado where do I look next

Use a radiator pressure tester, this will reveal any leaks quickly. Autozone does free tool rental, test system when cooled enough to open the cap without burning yourself! Antifreeze will stick to you and burn. Read the pressure rating on the cap, pump system to that pressure, it should hold steady! Also test the cap with the cap tester, if it's in the kit, caps also loose their ability to hold pressure, this will also cause antifreeze loss.