Check level in the radiator when cold. Do not just check the white plastic expansion tank. The radiator itself must be full to the top.
Check that the heater valve on the firewall opens fully when you put the dash control on full hot. Since this is partly electric, key must be turned on. I think the open position is with the cable retracted.
The dash gauge should warm up to about halfway and stay there through all driving conditions. Both heater hoses and the top radiator hose should be too hot to touch. The temperature of the bottom radiator hose isn't important. It will heat up if the car is sitting still and cool off when it is moving.
If one heater hose is hot and the other one is cold, there is low coolant level, the valve isn't open, or there is blockage in the core. The first two points should have been ruled out by the tests above. If a stop leak agent has ever been used in the system it is likely the core is blocked.