If you're burning a litre of oil every couple of weeks, you should definately notice blue smoke coming from your exhaust. Also, your spark plugs will definately be oil fouled.
If this is the case, there's a couple of quick checks you can do to help determine where the oil's entering the combustion chamber.
On a cold start, check for some good puffs of blue smoke that go away once the engine warms up. If this happens, chances are your valve seals are leaking. If the blue smoke doesn't go away at all, then you're piston rings are likely the culprit.
Another good way to determine if it's valve seals is to have a friend follow you while you're driving. Have him or her watch your exhaust as you do the following: On a long straight stretch of road, drive the car up to around 80Kmh or so. Take your foot off the accelerator and coast down to about 30kmh, then step on it, if there's a big puff of blue, you can pretty much bet your valve seals are smoked.