Individual coverage:  If you purchased individual insurance, read your policy.  If you lost it, contact your insurance company to request a copy.
Group coverage through your employer:  If you earned your disability insurance coverage as an employment benefit, or purchased it through your employer, you can ask your human resource contact or supervisor for a copy of the Summary Plan Description (“SPD”) for each benefit plan and pension plan, in which you participate.  Some employers allow you to access these documents online.  The SPD should contain basic information about your coverage.  However, there can be other documents that superseded the SPD if there is a discrepancy.  Also, your coverage may be governed by more than one document.  You can request a complete copy of your plan governing documents in writing from the plan administrator, who is usually the employer.  There are no magic words for the request.  Here is example language:

I request all documents governing my coverage under all group plans sponsored by {***INSERT EMPLOYER NAME***} including, but not limited to, {***INSERT BENEFIT TYPE e.g. short-term disability, long-term disability, life, health, long-term care insurance, and pension***}.  This request includes, but is not limited to, all documents to which I am entitled under ERISA 29 U.S.C. §1024(b)(4), such as the latest updated summary plan description, latest annual report, any terminal report, the bargaining agreement, trust agreement, contract, or other instruments under which the plan is established or operated.