How would I know if I am allergic to my deodorant?

The only way to test your hypothesis if the deodorant is causing it is to withhold its use. If the condition improves or does not recur after you stop using it, then it is possible that it may be it. Then try using it again. If the condition recurs, then THAT is a strong indication that the use of the deodorant and your condition MAY be related, or follow a causal relationbship. The same thing is done with contact allergies. For example, among construction workers dealing with mixing/handling cement. Upon consult, history reveals use of the cement at work with development of itchy skin lesions of both hands which is exacerbated while at work and improves dramatically upon taking a vacation leave from work. Then from this, one may make a connection between the two variables - the existence of the skin condition and use of cement as an initial step in the process of determining the cause of the allergic reaction. Of course, patch testing (see google), which is a definitive way of investigating it may be the next step to validate this.

I used a type years ago that gave me a reverse reaction. I just start sweating like crazy and I had no idea why. I stopped using it and I was fine. Maybe try another brand just to be safe.