Number one answer is always Head Gasket. The reason is that there are oil and water passages in close proximity, being separated by a gasket (which might just be flimsy paper or rubber, but is often steel or copper) sandwiched between two pieces of metal with (usually) different thermal expansion rates. This means that any time your engine gets far enough out of the proper heat range, the seal will fail and oil and water will begin to mix. Other problems will also result.
The number two answer is "oh, so you decked the block/shaved the head and replaced the head gasket... did you check the head for cracks?" Again, it comes down to the fact that the head is typically full of oil and water passages and the head will typically be the loser (structurally speaking) when something bad happens in terms of overheating. The block does have both oil and water passages but they're much farther apart (the block is primarily concerned with oiling the rotating assembly and the piston walls while the coolant passages are on the outsides of the top halves of the cylinders).
The third answer is a lot more car specific. Some cars (like the mazda bp) use a little housing that contains both oil and water flow to cool the oil with water. This almost never leaks, but it might be something worth checking out if you recently messed with it. Or maybe your car is equipped with a water cooled turbo... if the seals went bad, you might get oil flowing into the coolant that way.
There aren't special tools beyond those required to take apart the engine. External oil leaks can be spotted via dye and UV lights but that won't work here. Oil in the coolant is usually a prime diagnostic tool for head gasket woes. If it isn't your head gasket, you'll already have the head off, so you might as well drop it off at a head specialist to have them check it for cracks and warping. The most common causes of bad HG I've seen (other than overheating) were improper torque, improper orientation of HG, use of gasket maker when inappropriate, lack of gasket maker when required and severe uncorrected head or block warpage. If it is your head gasket, you should probably at least check that stuff with a straight edge (check straightness on either side of the cylinder line, across diagonally and across the top and bottom of the line). Also make sure you don't own a mk3 supra or similar car known for improper factory torque specs, etc.
One trick that might produce results is to remove the radiator cap and thread an air hose into the spark plug hole of each cylinder, one at a time (a compression tester will have the right threads), rotating the crank/cam assembly to close the valves for that cylinder and put it in TDC. Then dump a little oil and then 100 psi of air into the hole and see if it makes bubbles out the radiator hole. If it does, break out the wrenches and get to work on that head gasket. This might not find the problem though... you can get leakage between the oil and coolant passages without the leak intersecting the combustion chambers. You could put air pressure into the oil system, but you have a very high risk of blowing out seals on the engine by doing this. The combustion chamber is at least safe with high pressures (or should be, normally).
