Mostly the fact that their lip movements don't match up with what you actually hear-sometimes they'll even close their mouth earlier than supposed to.Overall you should be able to hear them breath in before singing the line and majority-especially if they have hard choreo will sound less stable.
Pre-recorded means just that you are hearing their live vocals but they are not the ones from the performance you are seeing so they are lipsyncing there.
This is a good example of prerecorded vocals-notice how they never lose breath or breath in deeply,how stable they spund despite having choreo like that and constantly moving