The exhaust gas temp is the best indicator of a lean condition.
Aircraft engines monitor EGT all the time to set the mixture. You would be amazed how high it can get.
There is no place to put a sensor but an IR unit could check the exhaust pipe below the manifold (thinner than the manifold)
exhaust temp at idle with heat gun are right around 500 deg at the collectors. 400 at each tube. again this is with a heat gun so.....
