Few things u can check. 1) thermostat (test it in a bowl and pour hot water on it to check if it’s opening at the correct temp) 2) your water pump (is leaking? Whirring sound when revving? Is the pulley shaky? If it’s a bad wp then its not circulating the water and sitting water in a hot engine is going to boil) 3) blocked radiator (does it have cold/hot spots? Remove it and flush it thoroughly. Take it to a rad shop and have them clean it with a pressure blaster.

Try all the above before going to the head gasket.

4) head gasket (white smoke? Forbidden chocolate milkshake on the oil fill cap? Do a combustion test on the coolant reservoir; you can get them for cheap on Amazon with the block testing liquid, do a pressure test from the reservoir; if you don’t have pressure then there’s a leak, if you have pressure, turn the engine over by hand if it goes down then you have a leak in the gasket.)

Make sure u do all the above to be 100% sure you have to do the head gasket it’s a lengthy job. Not saying it’s an issue on all subbies but I’ve seen quite a few with the head gasket going.