I imagine that it's amazingly difficult. Hiding your identity, especially when you're trying to be a completely different person, is very difficult. After it got into a habit, it might've been easier, but letting things slip is a natural thing for people to do. Masks are hard to wear all the time, eventually they fall off. After all, there is only one truth, and in time, people uncover it.
Poker face isn't that easy to keep either. Who can remember day after day, week after week, month after month to keep a smile on and not let their emotions seep through? Believe me, I've tried, and I failed miserably. For a short period of time, you can keep a poker face on, like when I'm lying to my sister about cucumber being spelled with a Q, but after a while, it will slip off for brief periods of time.
I guess the part that would matter the most about poker face (sorry, wandering off here) would be the eyes. Your eyes are silent, but they can speak out in a different way. It's like when Ran made chocolate, the people were smiling but the eyes were obviously sad. That's one of the reasons why drawing eyes is so difficult. Eyes truly reveal your emotions, and only professionals can keep their eyes happy for a long period of time.
I wonder what Conan thinks about besides deductions, the Org, and Ran sometimes. Anybody have any idea?