Determine units which are or can be detected.

[Ilias48rus 32]

Need some way to make system which will trigger every time some unit are spotted or in certain range (according to both units's types) with line of sight to another unit and returning both the units.
Guies not to easy, but meeting needebility of it not first time and saw few times others needed it too, if someone have solution or know how to make it glory forever.
Very appreciate any help.