Hi Smita,
The success of an onboarding program depends on the type of company, the nature of the business, and the current style of the onboarding program. A good feedback mechanism will give you a lot of pointers as to the areas of improvement required.
When it comes to the questions one may ask an onboarding specialist, the emphasis will be on the experience, implementation, and uniqueness of the onboarding programs that have been handled in the past. There could be other topics to explore, such as the key to successful onboarding, reasons for failures in implementation, evaluation methods of the program, etc.
Best Wishes