If you have a significant health impairment caused by mold, you'll need a lawyer to sue your landlord or other responsible party. In that event, the lawyer will have the mold tested for toxicity as part of the "discovery" phase of the lawsuit—that period of time when each side gets to ask for and examine the other side's facts.
For advice on lawsuits involving toxic mold, and how to assess liability, see the Nolo article Toxic Mold: Who to Sue. For help in finding an attorney experienced in mold cases, visit Nolo's Lawyer Directory, where you can view information about each lawyer's experience, education, and fees. By using Nolo's directory you can narrow down candidates by geographic area and specialty before calling them for a phone or face-to-face interview.
If you have mold-related losses (for a health-related problem or property damage), you might be able to sue your landlord in small claims court, if your claim is in the $3,000-$10,000 range, (the small claims court limit in most states). See the Small Claims Court & Lawsuits section of Nolo's site for details on small claims court, including how to do the following:
If your possessions have been ruined by mold and must be replaced, contact your renters' insurance agent immediately. Your renters' insurance might cover the cost of replacement or damage repair—depending on what type of coverage you have and the cause of the mold problem. (Your policy might exclude mold situations that develop over time.) Do not expect your renters' insurance policy to cover the costs of medical bills, however—you'll need to turn to your own health insurance for that (or, you can sue the landlord).
If you do not have renters insurance, or it does not cover your particular mold-related loss, you will need to sue your landlord as discussed above.
For additional information on landlord responsibilities and tenant remedies (including preventative measures) for mold and other environmental toxins, get Every Tenant's Legal Guide (Nolo).