The functional groups present in caffeine are amines, amides and an alkene group. By comparing a list of functional groups to the structure of caffeine, it is possible to find one alkene, two amides and two amines.
The four nitrogen atoms in the caffeine molecule result in either amines or amide groups depending on whether or not they are bound to a carbonyl, which is a carbon atom with a double bond to an oxygen atom. Amine groups consist of any carbon atom bound to a nitrogen, as long as the carbon atom is not part of a carbonyl, in which case these are amides. Alkenes are two carbon atoms with a double bond between them, which can be seen at the point where the two rings of caffeine meet.