Well, yes of course it is easy to tell the difference if you know what to listen for. On an acoustic piano, all the strings will resonate in response to a single note, referred to as sympathetic resonance. This is especially true when all the dampers are lifted. This is not true on a digital. That is not to say this couldn't be mimicked electronically if there were the market for this feature. I'm certain it could.
In fact it is so on newer digitals. My Kawai ES-6 does so.
Ken you are absolutely correct, as is ando in an earlier post. My Bad! Opened my mouth too soon.
I went back and checked my YDP-181 manual. Lo and behold, there is a button I never use called "Damper Resonance". I tried it out, and I find it a poor simulation of the real thing. It doesn't sound a bit like the resonance I get off of my soundboard and strings, and I still contend that on mine I can easily tell the difference. Maybe this feature is better in higher end digital pianos. I certainly hope so. I think I'll leave the button off.