911 Valve Stem seal Replacement
 

Is it possible to insert valve stem seals without pulling the engine?

Howard

Yes, the intakes can be challenging but it is possible.
I would only recommend it for one or two seals.
If more than two seals need replacing, pull the engine.

Valve stem seal
 

How do I know which seal is bad? The engine is a rebuilt 1972 2.4 with less that 1000 miles and sometimes smokes when started up and when sitting at a stop sign. There is no smoke when driving or down shifting. I figure the guy I paid to rebuild it failed to install new stem seals.

You may be able to tell visually. Sometimes the seals are broken or cracked. They are a pain because you have to remove the rocker shafts and valve springs. That is why you should remove the engine if you have to replace many of them.

-Andy

Spark plugs should show signs of oil consumption.
I would say that smoking at idle and start up are not generally signs of bad guides.
Valve guides generally leak on decel.
Take your car up to 4,500 rpm in 3rd and without pushing in the clutch allow the car to decel to 2,500 rpm. Now come back in the throttle and look in the rear view mirror. If you puff smoke the guides/seals are bad.
If you see smoke under hard acceleration the problem is rings.

Smoke at start up is rings and/or oil level too high.

These are general rules (a good place to start) and more research to accurately diagnose the problem.