The feel of it
Buildup is the film you can feel before you can see it.
Scrape a fingernail along your part and look at what comes up under it. Buildup smears instead of flaking — a pale ribbon with the texture of soft wax, sometimes grey where dust has settled into it. Push through the hair at the crown and the skin there feels tacky rather than dry. Hair goes flat by early afternoon, roots first. Dry shampoo buys an hour and makes the next day worse. None of it is dandruff, and none of it comes off in an ordinary shower, because an ordinary shower puts shampoo on hair and only accidentally on skin.
So a head spa for scalp buildup is a wash built the other way round. You lie back with your head cradled in a hydrotherapy basin, water running the whole session at a temperature agreed on before it starts, and the therapist takes cleanser through the scalp in sections — part, work, move on — with the skin visible to them the entire time. Warm water and unhurried minutes do most of it, which puts the session at the plain end of the complementary approaches to stress NCCIH describes. The rest is two hands on your head for 45 to 90 minutes instead of 30 rushed seconds of your own.
The Signature Head Spa runs 60 minutes at $80 and is the tier most first-timers book, because the scalp cleanse step sits inside it. Milbon scalp care is used throughout, and every tier from 45 minutes up to 90 ends with a blow-dry, which sounds cosmetic and is not: hair left damp at the roots is where the next film starts forming. For the shape of the session rather than the buildup question, what a head spa is lays out the format from arrival to blow-dry.