Since you are using ConvertXtoDVD I have to assume you are converting video files of various types to dvd-video. That is what the program does. The converted dvd-video may have problems that were introduced during this conversion process. Play the dvd-video output from ConvertXtoDVD on your computer before burning to a disk.
You may just want to skip through the chapters normally, but if you have a specific spot on the dvd you just burned that you want to confirm quality, you should play the dvd-video at that spot from the copy made on your hard drive.
Having the ability to check the output is the reason you should always save the converted dvd-video to the hard drive and not burn straight to the disk. Though I don't use ConvertX, I use similar programs, and save the video, at least temporarily, on the hard drive. I burn to disks using ImgBurn...a free burning program that I highly recommend.