depending on legislation of the National Aviation Authority you will find that some countries will allow training on experimental(homebuilt) aircraft.
However, this catagory of aircraft is certfied by a NAA but not to an international standard.
Training for commercial purposes on this catagory of aircraft is not permitted, this commercial purpose can be the training itself (the instructor is getting paid) or the aim of obtaining a commercial licence.
To cut a long story short, if you have any doubt that a commercial school you're attending has not got it's aircraft maintained to such a standard as to maintain continued airworthiness then your first stop is the chief engineer of the maintenance facility, if this is not satisfactory then you can approach the NAA to which the aircraft is(are) registered. This however will give you at best the last renewal date of the C of A.
If this does not satisfy you altogether, you can then request a visit from an Airworthiness Inspector stating the reason you wish this visit to be performed, ie, serious issues relating to maintenance.
Do bear in mind though when you go this route that it will be your word against the maintenance facilities paperwork......