So did you buy him as a normal, and think that he may be a yellow belly? Or did you buy him as a yellow belly, but are not sure if he really is?
If you breed him to an ivory and do make ivories, then he is definitely a yellow belly.
If you breed him to an ivory and do not make ivories, then he may not be.
If you breed him to a normal and make yellow bellies, then he is definitely a yellow belly.
If you breed him to a normal and do not make yellow bellies, then he may not be a yellow belly.
Hope this helps.