I came up with something a while back, posted it in http://www.gamesas.com/topic/1347565-no-papyrus-isridinghorse-function/page__p__20308648__hl__horse__fromsearch__1#entry20308648. It only works for the player, but someone also posted an idea that would probably work for NPCs. Kind of weird they didn't include a function for it though.
Detecting the player is perfect but I'll be running it from an alias. I check it out, see if it works. They also excluded IsBeingRidden from the script functions. I hope they add them in an update. At least they exist which means SKSE will be able to dig it out.
Alternately, you could try http://www.gamesas.com/topic/1354316-conditions-in-papyrus/.
Cipscis
Thanks but this is to do with 1st / 3rd person views on the horse - A much needed mod in its self! Is there something there that you saw that can help. I might be suffering from scripters blind spot.

What I'm trying to do is have a follower alias read what the player is doing (mounted /dismounted) and respond accordingly. Some as what we did in Oblivion for horse riding and evp'ing.