How can I test a wire for small holes?
Plastic insulation for the mains is quite thick so any damage has to be pretty macroscopic to cause danger. Electrons won't come spilling out of a pinhole like water from a punctured tube (at least at mains potential). 

Just inspect it thoroughly, bending it back and forth and look for any cracks or any visible copper. If you find some, shorten the cord to eliminate it. If you think you might have damaged the cord in some particular place, you may be able to just remove that end and shorten it a bit. 

I could tell you to dunk it into saline solution and check the resistance but I think that would be a terrible idea. Or you could connect it to a 25kV+ source and observe it in a dark room looking for a purple corona discharge (an even more terrible idea).