Firstly this is very unusual, especially with a European manufactured Underfloor Heating Pipe with the appropriate certification standards. For instance Floor Heating-Direct pipes are manufactured by U Tube Systems in Holland where we have a very high standard quality control and in house testing on each pipe production with KOMO, MPA & SKZ certifications as standard on all pipes.
We always recommend to put the system under pressure before the Screed is laid on top, this will highlight any issues during the installation stage and the pip[e can easily be repaired.
If there is an issue with the system after it has been installed it will be obvious as the system will loose pressure which can be seen on the gauges located next to the manifold. You may also see a damp patch on the subfloor where the water is leaking from the pipe. To locate exactly where the pipe is damaged firstly you have to make sure the sub floor is completely dry and then you can run each length of pipe individually port by port to test which length of pipe is at fault.
When the system reaches a high temperature you can use an infrared camera to detect exactly where the underfloor heating pipe is damaged, unfortunately although you can repair the pipe it is not something we would recommend as they repair patch can fail in future years, we would recommend if possibly to replace the whole length of floor heating pipe. As mentioned at the beginning it is very rare that a good quality water underfloor heating pipe will fail due to a manufacturers default and it is likely to be an installation fault, hence by doing a pressure test at the beginning is so important.