You can’t. Just like you can’t ensure that an animal from a shelter is healthy. There is a presumption that animals are fully screened for health issues before being adopted out of an animal shelter. A very basic medical intake is likely administered at a shelter, but that’s really about it. Unless something is visible, shelter staff really have no idea about the health of a pet.
If this is something that concerns you, we recommend you take your adopted pet to your local veterinarian for a wellness exam, just like your local shelter would recommend.
In addition, we recommend that potential adopters ask the current owner about the pet’s medical history and/or vet records. In most cases, you’ll find that people who adopt via Home To Home™ are able to gain a more complete health/behavior history about their pet as they are able to directly gather this information from the current owner.