Soon they will release Prime raids in Wildstar which are scaled up versions from the already existing raids. The original versions will be "Prime 0" and the first versions to be released are "Prime 1", potentially going to "Prime 2" next and so on.
Their plan right now with Prime raid releases is this order:
Y-83 P1 > GA P1 > DS P1 > RMT P1 > ??
They already released Prime levels 0-15 for dungeons so those might already be mixed up with normal logs.
Before entering an instance the first person to enter has to set the desired Prime level, not inside. Then at any point in time GameLib.GetWorldPrimeLevel() can be called to get the current Prime level of the zone you are in. This includes open world, existing raids that do not have primes, dungeons and so on. There is no difference between calling the API in a prime 0 scaled instance or in an instance that does not have prime scaling. They will both give you the result of 0. So there does not have to be any filtering for raids that do not have prime scaling yet.
As you mentioned you can just grab the Prime level whenever the players enter combat to detect the difficulty of the current fight.
Y-83 Prime 1 is on the PTR right now and is available to be tested. We have a few logs, including an easy-mode kill from a while ago. I'll also include a hard-mode kill from the live server from the same day.
Y-83 Prime 1 PTR logs:
Y-83 Prime 0 Live logs: