99 OBW is the EJ25D - the most notorious Subaru engine for blowing headgaskets. If you have *random overheating* (unpredictable, night, day, ambient temps, grades, idling, in town, highway doesn't matter), with associated loss of cabin heat (after the first minute or so of heat), one of the radiator hoses is cool, and/or bubbling in the radiator overflow tank....those are key headgasket signs.
Of course check the thermostat - use Subaru or the new Xacta version available aftermarket in stores. And make sure fans are on when it's overheating - you don't need a chart - are the fans coming on when the car overheats? Make sure radiator isn't clogged (usually very predictable).
If you start suspecting headgaskets do not do the "standard" tests most mechanics use, those are a waste of time on that motor. Never do a compression, leak down, or oil/coolant mixing test on an EJ25, they're a complete waste of time due to the failure mode.
The symptoms posted above combined with ruling out other things usually is a dead give away. A hydrocarbon in the coolant test is the best indicator you got, though I've even seen one fail that when they first start...