How should I diagnose a thick white smoke that smells toxic and not necessarily sweet coming from my exhaust at startup and tapering off as I drive in my 1989 Honda Accord Lxi in Hawaii?

SmokySometimes oil smoke will look more white than grey and if your valve guides are bad oil will pool in the cylinders and begin to burn at startup tapering off after a while.I would still suspect a head gasket issue luckily the cure for both is the same procedure which is removing the cylinder head. If your engine is consuming coolant as well as oil then definitely its the head gasket, sounds like you have a mixture of both.