Actually you should feel it immediately, that the bar is not centered or the grip is off. Or if doing benches, you know the grip is off right away, without the aid of a mirror. Try not getting into the habit a always using a mirror as a guide. Though, at times, a mirror to check the form, is helpful. Some will have video shots of side and read performances of a exercise, like squats and DL's. And than, later on, check their style and errors.
We use a very old and bent/curved in the middle squatting bar. It's perfect, because the shoulders/traps seem to fit just right into the center and the balance. This is a original straight York bar, that's probably been used by hundreds of guy's over the years. There are new bars, a bit curved in the middle, that are on the market. Designed only for heavy squatting.
Even with squats, the knuckling on the bar should be the first clue as to your best balance spot. Most use that guide. With DL's, or cleans, some spread the hands, from the inside of the collars and "walk" the them towards the middle of the bar, to find their usual gripping spot. If using chalk or stick'um with the first set you will know for sure your best gripping spot for doing more sets. Can also use tape to mark the bar, better for a home gym than a public one. Good Luck.
