LtJoker

I have a very simple question here. How do I check the line of sight from a point other than a unit's current location? I thought I did that by marking a waypoint and then hitting L for LOS, but that doesn't work, neither does T for targeting. I was somehow sure I had done this once, but I haven't played CM:BO in a while and I'm not 100% sure this was even a feature. Can anybody tell me if this ever worked and if so how, or am I missremembering things? Thank you