How to check player visibility?

[god]

I'm looking for a command fot testing that player is reachable for casting spells or another action... Could you help or point me?

LOS - line of sight is original namein ultima world...

for example:
I'm running in back my opponent and casting An Ex Por (paralyze) first when I'll cast it I want check if opponent is still reachable and my cast will success...

[Yoko]

LOS calculated by server using complex routines. they can be scripted, but not in public injection version.

the simpliest thing you can always check is only a distance to object, using uo.GetDistance(object)