The appeals court checks over what happened in dependency court to see if the judge made a significant legal mistake. To do this, the appeals court looks at the legal papers and transcripts. Transcripts are written "scripts" of everything that was said in court. The appeals court will not look at anything new; the appeals court looks only at what already was done in dependency court.
When the appeals court checks for a legal mistake, it asks a few specific questions:
Did the court do something that the law does not allow?
Did the court use the wrong law?
Did the court misunderstand the law?
Did the court make a without any evidence to support it?
The appeals court views the evidence in the light most favorable to upholding the ruling of the court. The appellate court will not reweigh the evidence; instead it will see if there is any evidence to support the finding of the judge.
If there was a legal mistake, the appeals court checks to see if the mistake was significant. If the case would have turned out the same even without the mistake, then the mistake is small or harmless, and nothing will change.
If the mistake was large enough to change the whole case, things might change; the appeals court might vacate (undo or cancel) what the juvenile court did, or the appeals court might let the person re-do the case.