Off topic question - How to tell why car is burning oil?

[767Jockey]

OK, off topic I know, but there's lots of auto talent on here so I figured I'd give it a shot. Car is a '94 Nissan Sentra, 114, 000 miles, 5 speed manual transmission. Until last week, no smoke at all - never. All of a sudden the engine is sometimes smoking badly through the tailpipe, sometimes not. It's definitely oil, it's blue/grey in color and smells like burned oil. I can't find any pattern to it other than when it does smoke it smokes worse on power application that when you lift off the gas. Could be rings, could be valve seals, what else am I missing? If it's an oil ring I doubt that a compression test would tell, no? What surprised me was the rapid onset, it sort of happened overnight - from no smoke to lots in the snap of a finger. If it's rings the car is junk, it doesn't pay to do a rebuild on it. Any ideas, and way to tell what it is? Anything else you guys can think of other than rings or valve seas and an easy way to check? Thanks!
(Sorry for the non-Cobra topic, but there is so many talented guys on here I though it would be worthwhile to give it a shot.)