1996 Mustang, 3.6 liters, split 6

Can you see the coolant if you remove radiator cap? If so, take thermostat out, hook hoses back up, just for testing. With cross flow radiator, drain the coolant down till you can see passages. Start vehicle, flow should be instant. You might be able to see or feel coolant swirl without lowering the coolant, but, if you leave thermostat in, it will have to get warmed enough to open.

You might be able to do the same thing with heater hose. Take hose loose from engine, not heater core. Just bump the engine, to see if coolant flow.