There's two primary situations where you see constructive possesson. 1.) The possession of an item not immedately on your body; and 2.) the possession of an item by having possession of the pieces sufficient to create the item.
Regarding the first:
"'Constructive' possession is commonly defined as the power and intention to exercise control, or dominion and control, over an object not in one's 'acutal' possession."
Examples: You "can possess an object while it is hidden at home in a bureau drawer, or while held by an agent, or even while it is secured in a safe deposit box at the bank and can retrieve it only when a bank official opens the vault."
Taken from US v. Maldonado, 23 F.3d 4 (1st Cir. 1994).
Generally, you have to have proximity to an object and control. Proximity without control generally is not sufficient. See US v. Jenkins, 90 F.3d 814 (3rd Cir. 1996).
So, the next question is what is an "intention to exercise control." Well, if the item in question is your property, you are displaying an intention to control it. Similarly, if you're the bailee of property, you're in control. Ex. I ask you if I can leave my M16 at your house. You're in possession because you intend to control that firearm. You intend to lay a supperior claim to the firearm to all but me and my agents.
Regarding the second:
This is really just an extension of the first concept. You have the pieces, you therefore have the control and a prima facie case of intent can be shown merely by your possession of the pieces to assemble the prohibited item.
There isn't really a codified constructive possession statute, as others have stated, it's something that has evolved in the common law. You won't be charged with constructive possession rather, you'll be charged with possession and the government's theory for the fact finder will be that you constructively possessed the object in question.
Moral of the story - if you think you're in possession, you probably are. Hell, you probably are even when you don't think you are. Don't be around illegal stuff or possess the parts to make illegal stuff.