There are many different types of mold, but only a few produce toxins or other irritants that can cause health problems such as allergic reactions and asthma attacks.
The Illinois Department of Public Health does not recommend testing as a first step. (http://dph.illinois.gov/topics-services/environmental-health-protection/toxicology/indoor-air-quality-healthy-homes/mold-faqs) Keep in mind that, if mold is visible, testing is not needed to identify what type or level of mold is present. Furthermore, testing is generally not useful in determining appropriate steps for clean-up, no standards or guidelines exist regarding acceptable amounts of mold and testing cannot determine whether health affects will occur.
However, if you decide to have your home tested, the IDPH recommends using companies that employ certified industrial hygienists. The American Industrial Hygiene Association provides a searchable list of its members at www.aiha.org.
When it comes to cleaning or removing mold, the IDPH notes that the source of the moisture that is feeding the mold must be identified and fixed. Otherwise, any clean-up will ultimately fail. If you want to clean the mold yourself, the IDPH website has recommendations. On the other hand you can hire professionals. The IDPH suggests searchable websites provided by The Institute for Inspection, Cleaning, and Restoration Certification (http://www.certifiedcleaners.org/index.shtml) and the Association of Specialists in Cleaning and Restoration (http://www.ascr.org).
Sellers can be held liable for failing to disclose mold, even if they are not required to do so by the Residential Real Property Disclosure Act. In Werderman v. Liberty Ventures, LLC, 306 Ill Dec 227 (2nd Dist. 2006), Buyer purchased a home that had a mold infestation and sued multiple parties, including the realtors, home inspector and the Sellers. The Sellers had not disclosed the mold and the Buyers did not discover the infestation when examining the home before the sale. The court found the Sellers liable for common law fraud, breach of contract and negligent misrepresentation, among other things.