Running Lean

1998 Jeep xj 4.0 I6 4WD
I have a minor suspicion that I may be running lean. I have had this engine in for about 5k. The Xj is a barn find. I have replaced engine, water pump, thermostat, radiator, hoses, plugs, leads, rotor, dist cap, dist, fuel pump, o2's, brakes and a few other things not relevant to the engine. Long story short I just have a minor suspicion it might be running a little lean. A pinpoint to any sensors, wires ect that could cause this would be nice. Just to confirm or subside my concern. Thanks everyone

Temp sensor.

OBDII scanner with real time data is your friend and will confirm your suspicion without throwing more replacement parts at it.

Thanks very much to both of you

Not to highjack... buut since we are on the subject, replaced oil pan gasket last night, battery was unhooked for a few hours while I was doing this. and I found a large crack in the exhaust manifold. it now is throwing a code for running lean, and slow response from bank 1 sensor 1 (front O2)
1999 XJ
4.0
200k miles
Just switched to mobile 1
Also a 5 speed
The other issue is I noticed on my starter the flywheel is eating a groove into it, warped flywheel? cuz I tried to wiggle it and it didn't have any play what so ever
And will that mobile 1 help clean some of the varnish off?

the crack in the exhaust is going to throw off the o2 sensor and show it sluggish. as it wont change from permanently lean

ok, so replace header and itll be good?