Patients are formally admitted to hospitals by way of filling out forms. On these forms is a space to note the patient's religious preference. A daily "census" (listing every patient who is in the hospital) is produced for in-hospital use by staff. Clergy are allowed to look at the census so they can visit their parishioners/congregants who are ill. Dubs, like all other religions, have an organized arrangement whereby assigned personnel visit the local hospital and review the census, just as all other religious groups do.
Confidentiality is on you. If you don't select a religious preference, they will list you as having none (and some dubs actually do this to avoid scrutiny). The census is not public information. To view it, you have to be authorized. Hospitals view clergy as authorized.
Sometimes church members (including dubs) will show up at the hospital to see a patient, having learned they are there through the gossip network. In those cases, the staff will usually come in to your room, tell you that you have a visitor and who it is, and ask if you want to see them.