In general, the most important demons are said to have a signature or seal, which is personal and generally used by them to sign the acts of the diabolical pacts. But those seals can also be used as a protection against them by a conjurer when summoning demons. Some grimoires like The Great Book of Saint Cyprian, Le Dragon Rouge and The Lesser Key of Solomon provide these seals.
Inspired by the Book of Revelation 13:18 the number 666 (the Number of the Beast) was attributed to the Antichrist and to the Devil. In early Christian times There were also used three letters instead of three numbers: FFF; F is the sixth letter of the Latin alphabet, and early Christians named the weekdays with the seven first letters of the alphabet to avoid calling them by the name of Pagan deities (although this criterion soon prevailed); Friday is the sixth day of the week; so the F became a good substitute of the 6, but this form of writing the 666 soon was forgotten and the numbers remained in their original sense.
A non-Christian symbol, like the pentagram has been considered a diabolical sign when looking downward. There are several forms of drawing a diabolical pentagram: just the star, an encircled star, or the star with the head of a male goat inside, being the horns into the upper points of the star, the ears into the side points, the beard into the inferior one, and the face into the pentagon inside the star; it can be encircled or not. If this last description includes five Hebrew letters inside the circle it is not a diabolical symbol, because those letters act as a protection against the demon. It has to be noted that the diabolical pentagram does not derive from the Pythagorean one but from the Babylonian and Celtic. Babylonians (as well as Hindus still do it) used to represent some deities with the right hand upward and the left downward, meaning respectively life and death, creation and destruction, good and evil, etc., and so the pentagram with a point downward was associated with the Devil (remember that for Christian theology all Pagan deities are demons and their symbols diabolical). The Celts had a representation of a pentagram (without the surrounding circle) that was said was the footstep of a ghost or a witch with one leg of a goat instead of human; the belief in the "witch's foot" lasted for centuries in Christian folklore.
A cross looking downward was also attributed as a symbol of both, the Devil and the Antichrist. This is a late symbol, and was probably derived from the pentagram and the same origin of things looking upward and downward the Babylonians had.
The trident is also a symbol of the Devil, and sometimes it is drawn combining a crossed line in the inferior part to form a cross looking downward, and so both symbols form a new one. This symbol can derive from Hinduism in which it is the symbol of Shiva. Shiva's trident (a missile weapon) sometimes is depicted with a crossed stabiliser that forms the same figure.