Do hygienists have training in detection of mouth cancer?

No: But they see enough mouths often to notice abnalities. But not enough to substitute for you dentist!

Mine does, but: The dentist always checks again. That's what these well trained folks do; look in mouths and distinguish normal from something to look into further.

Often: Often. Not all do. Make sure DDS evaluates you also. See oral surgeon or ENT doc if you have any of these symptoms: Persistent mouth sore, persistent mouth pain, a lump or thickening in the cheek, a white/red patch on the gums, tongue, tonsil, or lining of the mouth, a sore throat, hoarseness or feeling that something is caught in the throat that does not go away, difficulty swallowing/chewing.