valve seal vs piston ring oil leak.
 

ok. i know that you can sort of tell the difference. i found an old stang that has oil in the exahst when it starts up only. NOT while driving.

valve seals or piston rings?

is there another way i can try to tell? if its valve seals, no problem... but i am not interested in replaceing piston rings.

Compression test
 

Do a compression test. If all of the cyliders test within 10% of each other, then the rings are probably OK.

Rev

You have two fairly easy choices: Do a set of compression tests, or use a vacuum gauge.

If you opt to do compression tests, the only way to accurately determine if the rings are bad is to do it twice, dry the first time, and wet the second time. If the numbers go up significantly after the wet test, the rings are shot.

With a vacuum gauge, if the reading at idle is lower than normal, but the gauge needle fluctuates three inches on both sides of normal, your valve guides are excessively worn. If, when you quickly close the throttle after running at 2000 rpm for ten seconds or more, the gauge needle DOES NOT jump two or more inches above what it is at idle, your piston rings are shot.

Take care,
~Chris