So I'm doing Firelands right now, and my guild downed Bethtilac, but unfortunately I didn't help much. My vuhdo doesn't show anything different when people are Line of Site, so I try healing and it doesn't work. Is there anything I can do ?

Line of sight means that there is an obstacle between you and your target. Check http://www.wowwiki.com/Line_of_sight for more information

You might try to use the addon Healbot. In most of the styles offered with it, The boxes will fade when your team members get out of range. As far as LoS issues, There isnt much you can do. Beth isnt a very dependable fight to base your performance off of anyway. Especially if you are the healer assigned to go up top. A more uniform fight like Shannox is probably what you should test your abilities on.