?? Exactly. Unless its way off its hard for us to know. For most driving and drag racing, its close enough. Activities with lots of 50 - 90% full throttle is where it would be noticed. Pulling boats out of water, circle track, road racing...
Mike Urich recommended testing-tuning part throttle acceleration by "crowding" vacuum readings and timing the runs. More throttle should equal more acceleration. I've tried it with accelerometer and datalogger and its hard to do. A serious flat spot will show up though, just need the vacuum gage to verify whether it relates to power valve opening point or not.
Shrinker wrote that it can be seen in the exhaust with NOx reader - but how many dyno shops do you know that have one of those? So thats not an option for most of us.