Sorry, i just reread my post, if you see a chalky powder like substance on the plugs, ie. the grey white power on the plugs, then your engine IS burning the oil in the combustion chamber. This opens it up to rings, intake valve stem seals (although, as MR2QIK has already pointed out valve stem seals usually allow a puff of smoke on startup.)
A plug from an engine burning oil in the combustion chamber
If the plugs are clean, then you can safely assume it will be the turbo. If you wanted confirm, the best was would be to remove the dump pipe and look in the rear of the turbo for evidence oil oil over the turbine blades. Sometimes, but not always, turbos with the rear seal leaking will leak enough to leak past the clamp/bolts that hold the rear housing on and you will find burnt oil running down the turbine housing, starting from the core and running over the housing, which you may be able to visually inspect from under the car.
A plug from an engine NOT burning oil in the combustion chamber.
Hope the visual reference help a little. If it is burning oil, there are a bunch of tests that can be carried out, including a leak down test, and dry vs wet comp test to get an idea on the state of the rings, although both of these tests rely on the valves sealing correctly to isolate the performance of the rings sealing.
Also a bore scope can be used to check the bores, from which you can work out the condition of the rings as well.