Technique
The word you will find in the ancient texts is "stigma" and the meaning isn't very clear. According to C. P. Jones, it means mainly "tattoo". Branding was for animals. Except in some crual practices, as discribed by Juvenal about bad masters branding their slave to hurt them, the bodymarks of the Greco-Roman antiquity were tottoos.
Who was marked?
1. Some slaves were marked on the forehead with the abstract of a sentence meaning they were running away. This sentence still exist on tabellae, the slave wore sometime on a necklace.
2. Vegetius tell that the recruits should be tattooed with the pin-pricks of the official mark after they has been tested in exercices.
3. It seems that some initiated poeple of the Cybele cult were tattooed too.
For Greeks and Roman, bodymarks were meaning mainly barbary, slavery and animality, so don't hope to find a tattooed senator