Could be rings or valve guides. You can try a leak down test but if the valves seal well and the valve guides/seals aren't doing their job a leak down test will not tell you the condition of the valve seals. My mower does the same thing but the smoke is a little more black from the rich mixture of the choke being on for a few seconds during start up. Check the oil level and fill exactly to the full mark then keep an eye on the oil level. If you have a moderate to severe problem the oil level will consistently be low as a result of oil consumption. Is the spark plug getting oil fouled?