Facing would be easy: One dot of noticeble color on the direction the enemy is facing (only displayed if enemy is visible though).That's one of the initial ideas I came up with myself, and with your comment to back it up I guess it must have its merits. I was considering COV in favour of a single tile because it would allow players to determine the risk of being spotted. Of course if a guard is facing a wall an entire wall tile is colour-coded, which may look awkward. But I'll give this idea a try, and see if it works.
Cone of view is tricky however. Maybe it can have two ranges like in Commandos. Cones can start with 3 dots and increase in width every 2 dot (or whatever). If you don't have 8 facings (cones could be difficult on good ol' dotted RL screen) and settle with 4, cones should be wider. Enemy comes to investige if @ is in outer range, sees @ and raises alarm at closer range. Only inner range is displayed, outer will be double/triple in range. Colored tiles is definitely the way to go.Initially I'd like to go with a fixed range per guard, limited by shadow. Players on lit tiles are spotted from (say) 10 tiles away, or (say) 5 tiles away if they're in shadows, probably even less if the player is actively hiding. The cones would be anywhere from 90 to 180 degrees wide, using 8 facing directions.
Well, I can comment, that directional FOV was planned for AliensRL, but finally the idea was dropped as being unplayable, and suffering from the "lighthouse" syndrome.I can imagine it's absolutely unplayable if the player character has directional view. I'm now using circular FOV for the player, and I intend to use directional COV for enemies only. I'll make sure not to mention the game's "realism". ;)