How can i determine if turbo has failed or failing
You can hook up a guage and hose coming off the intake manifold, run it into the cab and put the machine under a load and watch the guage.. OR pull the pipes, air and exhaust, off the turbo and look for engine oil in the lines and inspect the wheels.. also the side to side play.. DO NOT remove the pipes while the engine is running or get your hands near the turbo when hot.. A spinning turbo wheel will cut your fingers off and a hot exhaust housing will lite a cigar..
According to your other thread (never a good idea to start 2 threads on the same subject BTW) the smoke is coming from the crankcase breather. If that's true it's not likely to be the turbo.....