If a heatercore is bad your windshield will fog up with a slimy coating, or will leak either into the cab or if the necks are leaking then check the firewall under the hoses.
I don't always trust pressure tests because it can cause more damage sometimes if your not prepared to do the repair right away.
Coolant leaks are a B*tch. Tough to find. First check the int cab, then under the hoses. Shampoo the engine well (covering all electronics) or have a professional do it.
Then once 100% dry drive it and park it on a flat parking spot to inspect for leaks while parked over night. If you notice a leak trace it to that area. If not it could be leaking internally like a head gasket. Id also inspect the piss hole on the water pump, all hoses,connections,radiaor,cracked overflow resevoir. Also a chance that the last person who serviced it may never burped the air out.
Coolant issues are pain. My kids hotrod, I replaced 2 thermostats and finally traced to bad t stats. I installed a fail safe and a guage and it has never worked better.
Start with simple stuff. But you need a very clean platform to start with.
If you live in Canada like me and oil the snot out of it like me...will be tough to trace.
Cooling system is simple. Just process of elimination. I do not recommend bars leaks,pucks or any other band aid fix. Those do more harm than good and a bugger to get out when you properly do the repair.
my 2 cents