The lone pair of electrons in Caffeine are shown in RED color in attached figure.
Details:
In chemistry there are two types of electrons.
i) Bonding Pair Electrons:
These are those electrons which are being shared and are involved in making bonds. They are also called as sharing electrons. In given structure all the solid bonds either single or double are made up of bonding pair electrons.
ii) Lone Pair / Non-Bonding Pair Electrons:
Those electrons which doesn't take part in bonding and are not shared between atoms. In given structure the lone pair of electrons are found only on nitrogen atoms (single lone pair) and oxygen atoms (two lone pair of electrons) respectively.