I think the only real way to see if player is outside of range would be to get the distance between player and the controlled pawn, and manually compare it to whatever your detection radius is set to.
Otherwise, as mentioned above, you can test if a perception update was a success or fail, and use the result to do other things:
This noodle block from an ai controller bp will print hello when it sees player pawn, and goodbye when it loses sight of player. Instead of printing a string, you could set a “CanSeePlayer?” bool on the blackboard or whatever.