How do I know if I am allergic to the pills?

Allergic reactions are extremely rare (.008%) and mostly related to misoprostol. You might not know whether or not you are allergic to one of the medicines, but in the rare circumstance an allergic reaction does occur, it tends to be very mild (hives).[1]

^{[1] Hausknecht, R. (2003) Mifepristone and misoprostol for early medical abortion: 18 months experience in the United States. Contraception 61:463-65.}