I also used alotta audible clues with my carbs: If things were backfiring through the air filters, then I was running too lean, if I was getting loud popping noises (along with puffs of black smoke) then was definitely too rich. But yes, the spark plugs are probably the best indication of how you're running. Another indication was how the engine reacted coming off of idle, if there was a little interval of no real power, and funky sounds outta the pipes, then the sucker were off, there should hardly be any transition from idle to good power.
