I am curious about what perks some of the followers have. I have seen some of the named NPC's on UESP have perks, but the regular followers don't have any listed.

I have used the PC to add perks and it is difficult to tell if they work. For instance I have added Savage Strike and had a follower for long periods afterwords never decapitate and enemy though they will do some of the other finishing moves. I have also added Lightfoot to followers and they still activate beartraps.

However, some followers like Brilyna and Aranea when adding augment frost perks will give a script 'Augmented Frost add to .... with one rank.' Does it only work when that script appears afterwords?

"AddPerk" and its papyrus equivalent do not work on NPCs. NPCs have only those perks they are given on their base actor record. Most NPCs do not have perks.

Thanks, I didn't think so for almost all cases. But I do think the ones you can add to Arenea the priestess of Azura work that say 'Augmented Frost added to Aranea Ioneth with rank 1'. I can tell they do as her ice spikes look different and prior to adding she can't kill a wolf in one shot, but afterwards she can. it also works on Brelyna or whoever that dark elf follower is from the mg quests. I think you can add that same perk to Mercurio as well. augmented flames works on all of them as well.

Aranea Ienith already has the Augmented Frost perk, and Brelyna Maryon already has the Augmented Flames perk.

Aranea is not having the power of her spells increased by your efforts; she has advanced enough to be able to cast Icy Spear, a much stronger version of Ice Spike.