Okay two questions in regards to turning undead.

Firstly, if you are evil and you turn undead, would characters automatically know you are evil IC?

And...if you turn undead, would you be holding up your deity's symbol? I vaguely remember reading something about it in the Player's handbook. It would be strange for a cleric of Mask to be holding up his symbol to repell undead.

So...thanks

When using the "Turn Undead" ability, an evil cleric will channel negative energy to awe/rebuke and command the undead. This is fundamentally different from channeling positive energy to turn or destroy the undead, and it is clear to onlookers that you are performing an evil act. In theory, since neutral clerics should be able to do this as well (a feature which may be added soon, stay tuned) it doesn't necessarily mean that you are strictly of an evil alignment, but it's not a bad guess.

You are also technically holding up a holy symbol when you use this ability, and invoking the name of your deity to reprimand the undead, but for our purposes this doesn't automatically identify precisely which deity you follow -- though certainly a cleric who shouts "Bow down, creatures of undeath! Submit to your new Lord Bane!" is putting on a more impressive performance than one who wiggles a holy symbol without announcing whose power they are channeling.

So in other words, tough luck for followers of Mask and Shar?

Clerics of deities who pride themselves on secrecy and subtlety, such as Mask and Shar, may find it prudent to act in a subtle fashion themselves. When travelling with untrusted companions, for example, such discretion may take the form of not channeling displays of negative energy to attempt to overawe the undead and bind them into one's service.