Quick/dumb question- how to visually identify a 340 vs 360 vs 318 (with a 4bbl)?

You can narrow it down some by looking at the vibration damper down on the crank. A 318 will always have the thin zero balanced damper, and the 360 will always have the thicker offset balanced damper.

Now, the 340 is a little more difficult to determine as it could have either depending on the year. The best way to determine the displacement of the motor is mentioned above, the numbers stamped into the block right below the drivers side head, right up in front. If it's a 340 it should have 340P in the first 5 charactyers, the "P" indicating Premimum fuel!!

Good luck Sherlock!! Geof