Currently if a Python toolbox causes an error on import, it shows up in the tool listing with a red x.
If it's a syntax error, fine enough, that has been handled in the interface. However, if it's a runtime error, there's a red X but no way to check it. The only way i can check it is to copy the .pyt to .py and try to import it.
As an example, I created an empty pyt in ArcCatalog and modified it slightly. This has no syntax errors, but I get a red X because the code fails when ArcCatalog tries to load it:
It would be great to have a tool or functionality that does a runtime check of .pyt files by attempting an import and reporting any runtime errors.