You are correct you shod see the face of Kallah once before the Chuppah to know %100 it is her. If you dint see it before and you saw her face after the Chuppah it enough to be a witness.
The Rishonim argue about how much one has to see to create a good Kiddushin.
The Mordechai writes that even if the witnesses did not see the actual giving of the ring it won’t nullify a Kiddushin if they saw enough that the can say for certainty happened. (For example if they saw the ring in the hands of the Chattan and blinked their eyes and it was on the Kallah’s finger.) We can rely on certain amount of evidence to create a Kiddushin.
The Rashba (Shut 780) writes only by Kiddushei Biah we don’t have to see the actual act and it is enough that we know they went into a room alone since anyway it is impossible to see the actual act, and we can be pretty sure it happened. Kiddushin with money though the witnesses need to see the actual transaction and it is not enough to figure it out. So rules the Rema (Even HaEzer 42:4) The witnesses need to see the actual giving to her hand or her property. But if they didnt see the actual giving to her hand, even though they heard him say, “Marry me with this certain object,” and afterward she walks out with it under her hand, it is not a marriage until they see the actual giving (Responsa Rashba Siman 680).
The Sefer HaMakneh understood there is not an argument between the Rashba to the Mordechai, son the Mordechai is discussing a case where there was extremely compelling evidence that the transaction happened and the Rashba was in a case where it was a little less clear.
Based on this argument the Poskim disagree whether you have to see the face of the Kallah.
The Mabit (1:226) writes that a man who gave Kiddushin in front of witnesses to a woman whose face was covered and when left and the never saw her is no a good Kiddushin. Who says it was even a woman ,maybe it was a man, or she was married, or his mother. We are in doubt and therefore everyone single women in the world is potentially his married to him, but we just can’t verify the transaction. Even if she would come forth and say it was her under the veil she is not believed since the witnesses have to see her to make a Kiddushin.
He adds in a later teshuva that even if he uncovers her face afterwards and then they see her he must do Harei At Mekudeshet Li Bmaot She Natati, , you should be married with the money I gave you already. And when should agree and if not she is not married.
The Maharit brings a proof or his father from the Rashba quoted above. However he concludes that there is difference between seeing the giving and seeing the woman. So differentiates the Ein Yitzchak that they must see the actual giving of the ring so they could testify, but seeing the Kallah’s face it is not for testimony of what happened it is just uncovering facts. So too for Gittin the Shulchan Aruch (Even HaEzer 120:3) writes that for the Bet Din to recognize both parties even one witness is enough. Even a women or a relative are good for this since it is just giving over facts not testimony. So concludes the Avnei MIluim against the Mabit.
What comes out is that her is an argument between Poskim if the witnesses have to see the Kallah’s face before he gives the ring so it can be a complete witnessing. Others say it is enough to see the giving of the ring but they do not have to see her face for the Kiddushin.
The Poskim also argue according the opinion that they do not have to see the Kallah does than only before hand but they have to see her afterward or they don’t have to see her at all.
The Shvili Dovid writes that even if the witnesses don’t see her afterward it is fine. However the Teshuvot Rashmach Gagon writes that even according those to who say you don’t have to see the Kallah before the Kiddushin they have to see her face afterward in order to be able to give testimony in Bet din if needed.
There is an argument between the Poskim if there is an obligation to look at the Kallah before the Kiddushin to identify her. Even if you say there is no obligation some say they at least have to see her afterward to identify her. Therefore it is of course better to see her face beforehand. Of course it does not mean to stare at her, It is enough to recognize her face enough that they may testify at a later date. If not then seeing her afterwards is enough.