Its not always a tell-tale sign of milky oil residue under the cap or bubbles in the radiator.
If it blows between a water jacket and the cylinder you'll more than likely get bubbles, but not always.
Damp exhaust gases (test by putting your hand near the exhaust while running on initial start up) is one way to determine if its blown through a water jacket.
If its between cylinders, then it'll run rough.
Have you tried taking the spark plug out and attaching it to the lead and earthing it to see if the plug is getting spark at all?
Better still, where are you located?
If you're not too far away I can come have a look if you want a second opinion?