The first place to start may just be the IBO website: International Baccalaureate Organization . (By contrast to the US Education turtle of a website, this one goes like a jet and has everything you're looking for right at your fingertips. We know this is a schools guide, but we couldn't help noticing...)
The IBO has a very strong system for setting up IB schools and making sure they get off to a good start, but thereafter do not particularly inspect or certify those schools (although they do continue to keep a close eye on schools using their Primary and Middle Years curricula). Instead, they feel the results speak for themselves. Therefore, parents should look at an IB school's exam results and numbers of students qualifying for the IB Diploma: if those numbers are poor or dropping, take a much closer look.
If a school is an IB candidate, that's a good sign...but not if it's been a candidate for a decade. Good IB exam and Diploma results do not in themselves tell you about the feel of the school or whether it's right for your child, and no one is looking into every cranny in the same way an American accreditation or Ofsted inspection officer does (not only the academics but also the governance and financial stability of a school).
But if scores look good, and you like the buzz of the school, there's a good chance you'll find a rigorous programme that will allow a fairly seamless transition from one IB school to another. That is, before the final two year IB Diploma programme. It is very important to note that, in spite of similarities within the curriculum, the two year course is usually regarded by schools as being fairly monolithic. In other words, not made up of identical sequential parts that students can pop in and out of, from school to school, at will.
Be aware that the IBO exists to set up curriculum and protocols and they are very good at what they do, but counciling or guiding parents trying to make this transition is not part of their brief.