Having said all of the above, I will now contradict a good deal of it in a short follow-up. If you get in the practice of level-matching AB comparisons, and of monitoring at infuriatingly quiet volume levels, you will rapidly start to develop an ear for fletcher-munson effects, and taking these measures will become less necessary.
This is where the "golden ears" business starts to kick in. You ears are the same, your hearing is the same, but your perception becomes better-attuned to the effects. This happens fast, like learning to detect an out-of-tune instrument, but it requires a certain amount of careful, educated, practiced listening.