Overheating at high speed / rpm

I own a 98 subaru impreza outback with 130k miles. It was running great until I
recently started a longer highway commute.
Overheating only seems to occur after the car has been driving for a while, and
at rpms above 3.5k (ish), ie. when accelerating to merge or driving up a hill.
I have a feeling it might be a head gasket leak, but am unsure, I just replaced the
oil and didn't notice any white coloration.
Since I noticed the issue I've been keeping my car under 3.5k rpm, but as soon
as I accelerate above that the thermostat pings to nearly max then back down to
middle repeatedly, it will middle out after a while of lower rpm driving.

Are there any additional ways to check if it might be the head gasket?

Thanks!