If you remove both lower dash panels (left and right), you can see the heater pipes coming through the firewall and going to the heater core. You can check that part of the heater system for leakage as well.
If the heater core is leaking, you should notice it inside the car. There has to be a kind of coolant smell and the (front) window(s) will get fogged...
The only way to get to the heater core is by removing the complete dash (steering wheel, console, and a lot more). It seems like a lot of work, but once somebody has done it before the repair can be done within 8-10 hours (Kebowers, were do you have your car serviced?)...
with a minor 600+k