Most of her upbeat performances both on tour or in an event. The difference between Beyonce and Brittney is that the latter doesn't use prerecorded vocals, but sticks to the original track.
I never understood why she lip-synced at the Oscars that time. It's odd.
But generally she just has flawless vocals which for most popstars, would suggest lipsyncing. The VMAs were not lip-synced, she just sounded great. You can tell because the breathing you hear perfectly matches her breathing on screen. You can also hear the mic sometimes when it touches her costume or when she sets it down to do the tantra-chair dance.