Testing Makeup for Allergies
With all the products used in these guides there is potential for allergic reactions. Certain common things may be unlikely, but paints, latex and other fx supplies have the potential to cause reactions. Large numbers of people are allergic to latex, and you do not want to find this out after you have just painted it all over your face.
Also various dubious products are often imported cheaply in the run up to halloween. I have some fake blood that stains the skin terribly. It's quite good, but I do tend to avoid using it on my face :-)
The most common way to test products for allergies is to place a small amount (a few mm across) on the inside of your arm. This skin is very sensitive, and is also not visible normally, so not so much a concern if you do get a rash or staining.
You can do multiple 'dots' for your different products, and it is sensible to do this a day in advance so you can be sure there is no problem.