AFR - no. You need a real wideband sensor to detect it and something to display it in real time or log it for review.
So, if it's just a code reader, you really cant tell anything unless the car has stored some codes. The car can store some codes and not activate a CEL.
However, if you have a data logger, then you should be able to see the fuel trims. If they are pegged, then you need a tune or hardware correction.
If we are just talking a couple of bolt-ons, run it over to your dyno shop for an A/F check. If you are running anything more exotic then you may want to install a wideband gauge or logger.
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