If Megaton is blown up, the hollowed-out rock will still be there, but the trees will not.A message just like the one displayed for an openable container says 'open hollowed-out rock,' therefore it is possible to walk up to every rock in the area and look for the message.Ask Dogmeat to find either chems, weapons, or ammunition.
If one is having trouble finding it, look for a rock surrounded by three trees that are much shorter than the rest of the surrounding trees. On the world map, the rock is just west of due south of the Megaton icon, about 3/4ths of the way down the grid square. The Lone Wanderer will usually be attacked by three to five mole rats or vicious dogs very close to the end. The location is around the back of Megaton. However, a closer inspection will reveal the rock is hollow and contains loot that's very useful at the beginning of the game. The hollowed-out rock, on a passing glance, appears to be just another ordinary rock.
