Two easy tests to check an engine to see if it's too lean.

Both are best done with the tank 1/2 full of fuel, though they work ok with a full tank as well.

Both are done with the engine running at full throttle.

First, pinch the fuel line briefly with your fingers or a clamp. You should hear (or observe with a tach, but I do it by ear most times) a slight RPM increase. No increase means you are right on the edge, best back off a click or 3. An RPM decrease means you are too lean. Of course, if you hold the pinch long enough, the engine will drop off and die as you cut off the fuel. But you should still hear an RPM increase before it drops off.

Second, lift the airplane and hold it vertically nose up for several seconds. Again, you should hear a slight RPM increase. Any drop in RPM means you are too lean.

Some guys who have 3 hands do both, hold the airplane vertically, then do a pinch.