The first option (a doctor's statement) of proof will suffice.
This, from IRS' Publication 502, Medical and Dental Expenses:
What Are Medical Expenses?
Medical expenses are the costs of diagnosis, cure, mitigation, treatment, or prevention of disease, and the costs for treatments affecting any part or function of the body. These expenses include payments for legal medical services rendered by physicians, surgeons, dentists, and other medical practitioners. They include the costs of equipment, supplies, and diagnostic devices needed for these purposes.
Medical care expenses must be primarily to alleviate or prevent a physical or mental disability or illness. They don't include expenses that are merely beneficial to general health, such as vitamins or a vacation.
Besides deducting the cost of the assisted living facility, you can also deduct part of your life-care fee, if any. This is from the same publication:
Lifetime Care—Advance Payments
You can include in medical expenses a part of a life-care fee or "founder's fee" you pay either monthly or as a lump sum under an agreement with a retirement home. The part of the payment you include is the amount properly allocable to medical care. The agreement must require that you pay a specific fee as a condition for the home's promise to provide lifetime care that includes medical care. You can use a statement from the retirement home to prove the amount properly allocable to medical care. The statement must be based either on the home's prior experience or on information from a comparable home.
