They only way to know that if you are in a rebound relationship is by communicating with your partner. If they are completely over their past relationship then they should give no reason to make you feel that way.
Only you can really answer that question. Here are some question that you can ask yourself to figure that out:
Are you over your ex?
Do you still love and/or have any feelings left for your ex?
Did you start your current relationship soon after the breakup with your ex?