Easy enough to test, disconnect the water lines from the turbo.

 

Simple, elegant. Love you. BRB.

 

An easy (old tech) test for a head gasket / water leak is to remove the rad cap at idle. Once the water pressure is released and the rad cap removed If you have a bad head gasket you will see lots of bubbles passing through the upper tank of the rad and possibly there will be enough air to push out lots of coolant out the rad cap (if the leak is larger). You should not have to rev the engine to see air. (For a small leak, this test works best if the thermostat is open so, be careful when removing the cap... ).

Steam out the exhaust from a bad head gasket is from the gasket allowing water to pass into the combustion chamber on the intake stroke. But, the compression stroke and power stroke have much higher pressure so, it ALSO blows combustion air into the coolant. IF you do not see bubbles in the coolant, it is quite possible your water leak is elsewhere. If its the turbo, then there may be some dampness in the compressor outlet.

 

As a former subaru owner with an early EJ25, can confirm the bubbles are easy to see in the coolant.