The first thing you want to do is look at the sword and ask yourself, "Does this look authentic?" You can usually tell when if a sword is authentic based on the first look.
I think we're done here...
But sometimes looks are deceiving, and you need to delve a little deeper. The second thing you want to do is unscrew the pommel.
The quickest way of knowing that a sword is crap is whether the pommel is fixed or screwed on. Again, you don't have to go much further than this to realize you are the proud owner of a piece of junk. But for the sake of investigation, let us dig a little deeper.
The next thing you will notice is that the tang is just a thin rod, instead of a proper tang. Normally a tang is just a little thinner than the blade itself, to promote proper structural soundness.
Also, bonus points if you noticed that the rod isn't even part of the blade. It's just welded on! This thing is going to snap the moment any kind of pressure is put on the blade.
So there you have it, an authentic terrible sword. Keep the steps that I've mentioned here and you too will avoid wasting your money. If you buy the sword from a pawnshop you can try to discretely unscrew the pommel. But if you're buying offline you'll have to trust more in the first step.
Fortunately I didn't have to pay for the sword I use as an example, since my cousin found it in her closet and gave it to me. And at the very least the manufacturers were kind enough to leave a third of the blade blunt. That way when the "tang" inevitably snaps I'll at least be able to wrap some sports tape around the lower section of the blade and still have some kind of weapon. It'll probably be more effective like that anyway.
At least I didn't have to pay for it, my cousin found it in her closet. And at least the manufacturers were kind enough to leave so much of the blade blunt. This means that when the blade inevitably breaks, I can wrap several layers of fabric backed tape around it and still have a weapon. It'll probably be even better than when it was in sword form.