Almost all states have a required test to prove grade level equivalency for all students. As education is mandatory for "school age" children; these tests not only prove capability but that the parents are actually doing the legal duty to provide education.
College entrance is NOT an issue. After comparing SAT/ACT scores ever university I know prefers home schooled kids over most candidates, including all but the very best private school students. The academics I have talked to say that home schooled students are both more likely to graduate and do better academically than their non-home schooled counterparts (although I haven't seen any specific numbers to prove it.)