The fact there is no verb "fier" in WR is due to the fact that the transitive use of this verb is not found any more, it's too old; but you will find it in a big dictionary, such as TLFi.
To tell what is the function of the word "se" you must consider whether there is a preposition used with the verb. If there is one then the function is COI, otherwise it is COD.
- Ils se parlent souvent. (parler à qqn, therefore COI)
- Ils se voient souvent. (voir qqn, therefore COD)
This has nothing to do with your example, though. The agreement is always necessary when the auxiliary is "être" except for pronominal verbs when the COD is after the verb and when there is a COI (ref.).
In the case of "se fier" the agreement is applied; however this string is not sufficient for making up a sentence, you must have a complement introduced by "à".
Here are cases of "se" as COI (you should have found them in the "obs" reference above).
Examples of "se" as COI:
- Ils se sont plu. (plaire à qqn)
- Ils se sont déplu dans cet appartement. (déplaire à qqn)
- Elles se sont ri de son erreur. (se rire de qqn)
There is however the following case in which the function of "se" in the second sentence is not usual (ref.); it is called COS (complément d'objet seconde). There are two "compléments d'objet" , "les mains" being COD.
- Ils se sont lavés. (action done on the subject, therefore there is agreement)
- Ils se sont lavé les mains. (COD after)
Here is a reminder on the agreement of past participles used with "avoir", the rule being that agreement is necessary when the COD is placed before the verb.
- Les oiseaux qu'ils avaient vus étaient des flamants roses. (agreement)
- C'était des grues; ils les avaient vues avec leurs jumelles. (agreement, antecedent of "les" is "grues" (bird, feminine))
- Ils avaient vu les flamants roses dans le marais mais ne savaient pas ce que c'était parce qu'ils étaient trop loin. (no agreement, the COD is after the verb)
"Elle s'est nui ", "Elles se sont promis de se revoir", "Ils se sont écrit tous les jours", "Elle s'est coupé les cheveux"
Here is the beginning of the entry for "fier" in the TLFi.
-
- Usuel. [Suivi d'un compl. prép. introd. par à, dans, en, sur
(littér.)]
You should get used to the abreviations.
- compl. : complément
-prép. : préposition
- littér. : littéraire (in the literature, not everyday language)
- introd. : introduit
It is the same for "parler" (TLFi); look up "II B".
It can be time consuming to go through the several lignes of an entry but the information is often somewhere among them; you must scan the entries carefully.