Which things should I ask the hospital caseworker about hospice for my husband who's dying of lymphoma?

Ask if they can help. Sorry for your husband's illness. Hospice can be a source of great comfort both for you & your husband. They can provide respite care for you, so you can have some personal time while he is cared for he can have another volunteer come & visit. Hospice services typically provides all medication that is needed, but not for room and board. Typically the hospice nurse can answer all these questions.
Tell them your goals. A hospice should be able to meet the goals of the patient and family - a skilled hospice team should elicit these goals without even asking. Tell them your concerns: pain, restlessness, how the disease will progress, equipment needs, fears, any question. If the hospice is able to address your concerns, pick that hospice. If a hospice is pushing their own agenda, choose another hospice.