I am wondering, not being a distiller, where "sweetness" comes from. Does "sugar" in the typical sense come off the still typically? I would think not but don't really know. Esters, and other congeners do but they aren't or at least don't start out as sugars. And yet some new make can seem to have sweeter taste than others. Do congeners convert to sugar or just stimulate the "sweet" taste buds the same as sugars?
I have presumed most sugar comes from the sugars in the barrel of the wood that are acquired over time by expansion and contraction of the distillate through the wood, which with most things is charred in part to help pull the sugars to the surface and caramelize them.
So if you examined new make whiskey in a lab would you find "sugar"? Or would you have to know to look for different types of sugar like xylose, glucose, lactose, etc and depending on what types of sugar you find can you speculate what their source is? Presuming they don't get broken down over time!