What kind of visual sign does would see if an engine has been running lean?

 

Spark plugs insulator noses will be very white.
Also when coasting you make hear popping in the exhausts, exhaust stinks, may smell like rotten eggs . white spark plugs, the plugs may even have small spots on them, spots are not good ,may be piston particles.
carb back fires are normally a lean condition or a vaccum leak. a lean carb may surge a cruising speed with steady throttle, lean can also cause over heating.
to fix......verify before changing the carb. check your engine grounds, you need to have a ground from block to body, next check charging system and battery, next check the ignition , and timing.....if any of these are not right you will go nuts trying to fix the carb... take a few minutes and save a few hours.....
heres a couple quick tricks to get a general idea of a lean condition...
if it is lean.... you can richen it a little by closing the choke a little bit by hand, or use your hand to restrict the air flow. engine will respond by picking up idle and/or smooting out.
smelling the exhaust is the easiest.
last.....watch the spark plugs... white after running is no good.. actually dangerous to pistons. again too lean.

 

babbah is right on, you may also get over heating, cherry-ing of the cat, holes in piston, but those are harder to see. learn to do a good plug-chop and read the plugs can tell you a lot about the engines tune.

 

Thank you for the inputs. To make the long sthory, I am helping a friend with his 79. several exhaust mounts bolts shared after discovering a broken exhaust manifold (drivers side). Both mounts on the 3 and 5, 4 and 6, shared and one had to be cut off.

so the heads had to be removed after unsuccessful attemp trying to remove the broken piece while intsalled. During the process of removing parts, I discovered the vacuum "T" fitting on the back of the manifold was lose. Hence he was ingesting excessive air, then the intake manifold (about half of them) were lose also.

The photos are the valves and what they look like in the clinder head. I was trying to explain to him what a lean condition does, to be honest I have never seen the affect on the hard parts. I am aware on what it looks like on the plugs, not on the valves or on the heads.

 

explain to your friend that lean means too much air with the fuel. just like an oxy-acetylene torch, add more oxy, lean it out, it gets hotter. then you can weld your valves to the seats, snap the heads off, burn holes in the pistons, etc. looks like you caught it before the real major damage was done.

glad you figured out the root cause.