Words in a manuscript margin may attract attention precisely because they sit outside the main block. Their position is observable; their author and date may require much more evidence.
Describe the relationship first
Is there a connecting mark, a visible insertion point or merely nearby writing? The TEI guidance for additions offers vocabulary for representing added material, including its location. Such description does not by itself settle which person supplied it.
Keep handwriting judgements provisional
A different-looking hand or ink colour in a digital image can prompt a question, but it should not become a confident chronology without specialist support. Image conditions and the manuscript’s material history can complicate a quick visual impression.
In a study note, give the shelfmark, page reference and the edition’s account of the addition. Then state your own observation separately. This makes a margin useful evidence without turning every annotation into the ancient author’s private explanation or a later reader’s opinion merely because one interpretation suits your argument.