but who checks who your old FI was
Its not that difficult to check. 25 hours dual on a SEP is required to get a JAA PPL. All instruction on such aircraft will be with an appropriately qualified instructor whereas hours on a Microlight or SLMG most likely won't be, so the aircraft type is the clue. It is the applicants responsibility to present the relevant evidence and is subject to a large fine if they deliberately falsify the evidence.