Daniel Jackson explains Goa'uld iconogaphy in a Stargate 101 video.

All Jaffa have a mark on their foreheads signifying the Goa'uld to whom they are in service. For most Jaffa, the mark is simple black ink. However, higher ranked Jaffa can be given more ornate, ostentatious tattoos. Jaffa in high standing have been known to be marked by a silver tattoo (for example Fro'tak), and the First Prime to a Goa'uld are given the gold embossed mark of their respective god. These tattoos are applied by cutting into the flesh with an Orak knife and pouring pure molten gold or silver into the wound. (SG1: "Family", "Bane")

The tattoos are obviously very painful to apply and very difficult to remove. (Del'nor burned the symbol off of his son Rak'nor, leaving an ugly mark). However, after Apophis took over Sokar's army he was able to change Sokar's mark to his own, this might indicate that the Goa'uld can remove the mark but keep the method well hidden. Priests and priestesses have been shown to wear marks of gold paint. Below is a list of known marks and the Goa'uld to whom they correspond:

Known Goa'uld symbols

Notable Jaffa marks

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