1991 Nissan 240SX Smokes ONLY on idle

If your 240SX is turbo charged then I assume you are running a SR20DET engine which you imported from Japan. Unless of course you live in Japan, which might explain the Japanese screen name. Either way with a name like jisatsu (suicide) I doubt many people want to drive with you.
Anyway if you have that engine then my guess would be that your problem is due to an oil leak in the turbo. Of course you should probably still rule out the other issues you mentioned. First inspect the spark plugs and do a compression check to rule out a blown head gasket. You could check the valve seals next but I doubt that's the problem. If it were bad valve seals the engine would only smoke for about 30 seconds after start up and then would be fine until you started it up again after letting it set all day allowing more oil to leak past the valves and into the cylinders. If it were rings on the other hand it would smoke all the time. Unfortunately I'm not sure of a quick way to know for sure if it's indeed the turbocharger. If there is an oil leak in the turbocharger though I would think it would take at least 10 or 15 seconds for it to heat up enough for the oil to start burning and producing smoke whereas oil in the combustion chamber would smoke immediately. This is just a guess on my part but if true it would be a good way to rule out turbos as the problem. Also even if the oil is coming from the turbo it doesn't necessarily mean the oil seal is bad. I would first check to see if the oil return line is clogged. Remember, if your engine is an import it probably sat around unused for several years before you got it and put it in that car. There are also many other things that can lead to oil in the turbos besides bad seals.