Every time you approach a rappel that is already rigged, you should inspect the anchor and rigging before using it. There have been too many deaths from anchor failures that were not properly inspected. Just because another group used the anchor in the past does not mean it is still safe. Here are a few things you should check:
If the anchor is questionable, you should consider replacing it with something safer.
If my group is ever in a canyon and we aren't able to fully inspect the anchor and rigging, for example a deadman anchor that is buried, we have one member of the group act as a backup meat anchor so that if the anchor fails while someone is rappelling, the backup anchor catches them. You don't want the backup taking any weight unless the anchor fails, so you're certain the anchor can take the full weight. I recommend having the biggest member of the group or the worst rappellers go first while the anchor is backed up then having the smoothest or lightest rappeller go last.