The best check-boxes on the market not only check the tested value, but also verify that tester output voltage and current are sufficient to comply with the requirements of BS7671:2008+A1:2011 Section 612
The market leaders are Kewtech FC3000, Seaward Power Check 1557, Megger MTB7671 or Transmille 2080, of which the Seaward and Megger versions are in my opinion the best. The Seaward Power Check 1557 oozes quality and is able check PAT testers as well, but at a premium price. The Megger MTB7671 is not quite so well built or functional, but it's easier to use and marginally cheaper.