Like the good Dokta said, Take it for a strap and shut it down at full noise(a "plug chop"), not a real good expierience for the turbo bushes, but still a better way to examine the plugs. Also a bit hard with the engine apart.

Another thing to inspect is the exhaust valves. When the engine runs lean, it creates a lot of heat, the exhaust valves copping a lot of it. How are they looking?

On the dyno with a sniffer is the safest bet.