Vocal recording sessions tend to be rather long. After recording and going through the vocal takes countless times, you start to lose perspective. This is why breaks are essential in recording.
My favorite quality test for recorded vocals is the (notorious) morning test. The idea is simple: first thing in the morning, listen to what you’ve recorded on the day before. If it still sounds good, it probably is good!
A pair of outsourced ears can be a good aid as well, but they’re not always around.