On my Open Pros and other Mavic rims, the heavy spot is actually opposite the stem... I assume the rim was welded there. Don't know about other brands.
When the bike is suspended, rotate the heavy side to 3 or 9 o'clock. (For a rear wheel, you have to take the chain off the cassette, or just take the wheel off and hold the axle in your hands). How fast does the heavy side fall to 6 o'clock? If the bearings are in good shape and adjusted right, it's perfectly normally for it to slowly rotate down, and oscillate a few times like a pendulum. If the heavy side drops fast, then it is unusually heavy.