Spell casting in HARP (in the core rules) is an obvious process. You always know when somebody is casting a spell. Well, you do as long as you are facing him....
In combat, you need a Combat Perception roll to figure out that a magic user with whom you are not engaged is casting a spell.
However, unless you take a skill like "Spell Lore" you won't be able to figure out what is being cast until it is done. So it does not matter what spell the foe is casting (and no, you do not need to know for counterspell), you just have to decide you are going to attempt to counter it.
When you declare a spell, you MUST declare what scaling options you are using -- no changing your mind after casting as begun.
Instantaneous spells - in general you can cast these while performing other actions, and they can be declare at any point up to the dice being rolled (at that point, the attack has ALREADY BEEN MADE and it is too late). I also do not declare monster OB/DB splits for parrying (I do make a habit of writing them down, so that I can show players later what the values are if they ask -- and I allow players to write their down as well, showing them when they take their turn -- no changing after decision made though).
The big exception on instantaneous spells is other spells. You cannot cast an instantaneous spell while casting another spell. You either don't cast it, OR you MUST STOP casting the first spell and then cast the instantaneous one.