Simply pull back the carpeting to the right of your gas pedal, remove the plastic cover (easy) and there you'll see the 2 tubes that go from the firewall to the heater core. If the heater core's leaking, you'll see evidense of coolant there but if you still need more proof, check this with a cooling system pressure tester.
BTW, it's easy to replace the heater core in a X70 and does NOT require dash removal.