You can't always know, really. Especially in Europe, airspaces can be rather complex, and there may even be vertical boundaries. There are a few thing you can do to make better educated guesses: for example, SimBrief will mark FIR boundaries in your OFP flight log, and those often align with handovers. For the UK, this page is tremendously helpful: it tells you that when you fly from Belgium into EGKK, you will most likely enter UK airspace at VABIK or KONAN, and you will be talking to LON_S_CTR (or LON_SC_CTR, or LON_CTR, depending which of these are staffed). VABIK or KONAN will be your airs