Just drop the V band off the turbo and check for any end float in the shaft. There will be some lateral play, but if you can feel any end float, it's a fair sign that the turbo is on it's way out. Also excessive oil in the inlet pipework can show up a failing turbo seal.
Failing stem seals tend to smoke the most when you first start up, or on overrun when there is a vacuum in the inlet, so you could try getting someone to follow you at highish revs then just close the throttle.
You can do the stem seals with the head on using the right kit. I would recommend if you are doing them though, to just buy a decent quality head gasket kit which should have all the gaskets and seals you need, and do it the old fashioned way. The kits are only around £100 or so. Take the head off, give everything a good clean up, re-lap the valve seats while you are at it. You should end up with a smother running engine and gain a few BHP back too.
Edited by ayresyy, 13 June 2019 - 07:07 AM.