I know driving any car hard, and with over 100,000 miles, will cause wear.
My questions:
1. What is most likely the cause, worn piston rings, or valve guides?
2. How can it be diagnosed without tearing into the engine?

 

compression/leak down testing can certainly give you some data. Valve seals are typically a mileage consumable but can go earlier than expected depending on the driving conditions.

 

Have a buddy drive behind you. If tour car smokes under acceleration it's the piston rings. If it smokes when you abruptly release the gas, it's the valve guides.

 

What's excessive to you? More than a quart every XXX miles?