The only way to truly assess the qualities a component possesses is to listen to it long term. By long term I mean more than 5 minutes in a strange room. Several weeks is a minimum, though certain qualities generally manifest themselves within a few hours. Why so slow? Because first impressions are notoriously bad and can't be relied upon.
The best judge for me is if I want to listen more and more. Middle of the road componentry doesn't wear well after time. If listening time goes down after a few days it's not good. OTOH, if I can't wait to get home at the end of the day to turn it on - it's good!
Marty
I think this can be individual to some degree.
I believe that if you absolutely "know" that this component isn't right for the system, it is something that usually will be
detected within say 5-15 seconds.
If it's just "wrong" that decision comes very fast.
If it is something that sort of have to "grow" on ya, It might be that one at first really can't put a finger on it or...
It's like falling in love really. You have those crushes... Wow! Great bass! Then it becomes apparent that the bass is something that is disjointed from the rest, it's great, but it's not complementing the midrange..
Then there are those slow emerging feelings that build over time, and is well, love...
Everything is cut from the same cloth and the cloth, well, it slowly comes to rest over the smooth figure that can be seen underneath.
Clinging firmly to every Nuck and cranny and behold... beauty! The beauty being the soul of the music, of course...
Now that is how it's suppose to be you know!!! All you need to do then, is to just reach out and lift that veil, that cloth... now that you know where to look... (Oh, poetic tonight!!!

)
The veil is the old component, of course..

Basically, I sort of see sound... It's hard to explain, but when adding a component I can see in my minds eye how it sort of either
distorts the sonic "image" or makes it clearer...when I hear it...
Thats how I judge sound. I see it. weird I know, but there you go.
Imperial