I've always used a stand-alone DVD player for testing. We have a few different models (Sonys, Samsung) around the studio. If they play on any of these, we can confidently tell the client that it's working fine on our end and suggest trying a different player on his/her end. We also had a problem where our client's client was having problems playing discs, only to find out they were having problems playing the copies they made by dragging and dropping the contents from our discs to ones they sent to their client. Don't assume anything.