How can a person know they are allergic to Botox if they've never had it done ?

Allergic reactions to Botox are very rare.  One of the reasons it is so popular is because it works so well for the majority of people.  I encourage you to discuss your concerns with your injector before having treatment.

The possibility of having an allergic reaction to Botox is extremely rare. In all my years of practice, I’ve never seen a patient have an allergic reaction. There is no allergy test for Botox, so if you are still concerned even knowing how rare it is, it may be better just not to do it.

Hello, Thank you for your question. The risk of having an allergic reaction to Botox is extremely low. Should this be a major concern to you, it may be best to avoid this procedure all together. I recommend you book a
consultation with a qualified injector, to discuss your options and concerns. 
Take care.

There are no known allergic reactions to Botox, I suggest a formal consultation with an experienced cosmetic dermatologist to determine best treatment options for you. Best, Dr. Emer

There have been no reports of allergy to botox.  There is evidence of antibody formation after injection, but these only lead to decreased effectiveness of the product in rare individuals.