I would think a sealed commercial cord would always be wired properly, but you could always test.
You can test any number of ways. Check both ends for continuity, pin to pin, or plug it in and check for 120V power between neutral and ground (should be none). The easiest is to probably just use a small neon outlet tester before you plug in. If you are unsure of any of the outlets or cords, the best place to check would be right at your welder cord, that's why I built mine right into the plug. Things like two extension cords both with swapped pins will test fine when used together, but bad used one at a time. Such are the issues with being mobile and using things that are not yours. Probably one of the big reasons that most professional welders never use site power and bring their own generators. You never know what you will run into. But if you do a few tests, you should not have any problems. You could even build a tester into your pigtail adapter if you want.