How can I test if I'm allergic to a face wash without putting it on my face?

I bought a Neutrogena facial bar a few days ago, and I used it when I got home in the shower. After I got out, my face felt like it was on fire! The thing is, I used different things. It wasn't the first time I used that kind of bar, and I used it before without any problems. I don't want to use it on my... show more I bought a Neutrogena facial bar a few days ago, and I used it when I got home in the shower. After I got out, my face felt like it was on fire! The thing is, I used different things. It wasn't the first time I used that kind of bar, and I used it before without any problems. I don't want to use it on my face again, in case it really is the bar. Where can I test it out, to see if I'm allergic to it or not? Like, skin that would react the same as my face?
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