Potential acquirers are usually sincere in their intent when conducting due diligence, but there are always those with bad intentions and some who are simply "tire kickers."
Some indicators as to whether they are serious are their acquisition track record or willingness to mutually exchange confidential information. Real prospective acquirers are usually willing to show how they will structure and finance a potential deal. There may be other anecdotal signs, too, such as the size and makeup of the buyer's deal team and how much time and money the prospective buyer is spending on potential deals.
The release of sensitive data (for example, strategy, facility tours, key customers, formulations, software code, etc.) should be phased until there is confidence that the buyer is serious about purchasing the company. Keep in mind that you are marketing your business to be sold, so you should share important financial and operating information in order to pique the buyer's interest.