Valve stem seals will give a puff of light blue (could look grey) smoke upon start up from oil settling in the cylinders. As was stated earlier, this iss likely the cause of your smoke.
As for the head gasket, check the coolant reservior for signs of oil in the water. Chances are if the head gasket is heading south, you'll spot it early on at the coolant reservior. Unless if you have a catastrophic gasket failure (yes they happen).
In the absence of oil in the coolant, you may see coolant usage, where you have to top off the reservior every couple days. This would indicate a coolant leak into an oil passage or into the combustion chamber (could even leak coolant externally at the head to block joint). Do you have any coolant in the oil? If you do you'll have a chocolate milkshake looking substance on the dipstick.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.