One is encountered at the GNR plaza, for the main quest line. One is encountered East of the GNR plaza at 'Takoma Park' One is encountered at the Jury Street Metro Station, One is encountered INSIDE the Capitol Building, One is encountered in Evergreen Mills, locked behind a cage.
The GNR Plaza Behemoth is spawned after a wave of Super Mutants are killed, and will target BoS NPC's over you. It is reccomended you climb the junk and shoot it from the balcony above the actual plaza.
The Takoma Park Behemoth can be damaged with a carpet bomb strike, located on top of an over turned truck.
The Jury Street Metro Behemoth spawns when you take a teddy bear from a shopping cart cage, although there are instances where this does not happen, I rested there for an hour, and turned around to a face full of fire hydrant, for example.
The Capitol Building Behemoth is in the middle of the huge room, fighting Talon Merc's, and the player can easily clear out the room using the Experimental MIRV launcher.
The Evergreen Mills Behemoth is locked inside a cage, and cannot hurt you unless you damage the generator outside it. This Behemoth has no lootable items, so actually destroying the generator is not reccomended, unless you like chasing big hulking monstrosoties. It WILL try to run.