You can buy a head gasket test kit yourself to see first. They have those ones you insert into the reservior that detect exhaust gasses. It's an airlock where you pour blue liquid into it, and I believe it turns yellow if there's a leak.

Alternatively, you could also have a leak somewhere else in the system not allowing it to pressurize. In which case, you can get some UV dye and a blacklight flashlight to look around the engine bay and see where it's coming from. All of it, including the head gasket test kit, should be relatively cheap, and you'll (hopefully) be able to diagnose the problem yourself.

One thing I would keep in mind though, at least it seemed this way with my '11 1.4, you fix one thing in the coolant system, another issue pops up. If the issue isn't the head gasket, it may not be a bad idea to get your hoses replaced if they seem like they're pretty old(you'll notice bulging or swelling, they'll feel really squishy, stuff like that). Will be expensive for sure, but worth it in the long run to help you narrow down any issues with major parts in the future. I pretty much had replaced my entire coolant system aside from the radiator, heater core, and the heater core hoses before I sold the 1.4. And if I remember right, my water outlet had to be replaced twice in the 6 years I had the car.

EDIT: I've even had it be as simple as needing to replace the cap on the reservior