To gauge what perfume she might want, try to drop hints without giving away your plan; telling her how good she smells and asking her what she is wearing.
Another suggestion would be to go in the bathroom and look for yourself while she is out of the house. You can either take a picture of the bottle or you can make a mental know if you are sure that you would not forget. If you do not live together, you can do that as well but make sure you excuse yourself to the bathroom when you are over for a visit so you can have a free minute to take that picture or make a mental note.
Mother’s Day is a special day. With the couple of suggestions mentioned on how to plan the best surprise gift, you can be sure to have a romantic day with the mom in your life.