How do you know if you got the fuel mixture set correctly on a carb?

How do you check or can you tell by just how good the car runs?

 

You can check the plugs for a rich condition.
You can get a wide band o2 sensor w/ a gauge.
Some guys change jets & other adjustments, run the car down the 1/4. They keep a journal of the times till the times start to drop off. This is done at a test 'n tune day at the track.