Form, pin, and section are your clues.
The cross-section of the hoop, the way the pin is attached, and the constriction at the pin rest are all datable features. Compare against published assemblages rather than guessing from the general shape.
Annular (full ring) and penannular (broken ring) brooches are the great medieval families, joined by smaller frame shapes — lobed, star, and heart forms. Many carry inscriptions: religious formulas and love mottoes in Lombardic or black-letter script are themselves datable.
An integral pin loop (cast as one with the hoop) reads differently from a soldered one. Note which you have, and look for wear where the pin bears on the frame — genuine use polishes that spot.
Use a reference — Egan & Pritchard’s Dress Accessories is a standard for c.1150–1450 finds. Record the hoop section, pin construction, and any legend letter by letter, and let “probably” stand where the evidence stops.
Identifying brooch and fibula types