Home checks are usually seen as compulsory practice with most rescues. It's just a case of checking that the setup you have prepared for your chosen piggies is suitable, particularly the size and location of the cage/hutch, and making sure that any necessary protection against the elements and predators have been considered.
The home checker will try to get an idea of your knowledge of guinea pig care, may ask you a few questions about what you intend to feed them and your ideas for exercising/floor time for the piggies, just to make sure that you know what you're doing when you start out with piggies. If you have done your research and have got a good basic knowledge of guinea pigs, it would come across quite obviously in the large majority of cases if the setup is appropriate.