Whilst it may make you appear a little geeky, cabin crew will in my experience ALWAYS find and advise the registration ("tail number" on US carriers) if you haven't been able to see it. If I haven't been able to find the info for myself when I board, when I board I usually say "what airplane are we on?" to which they reply "747" (or such). I then say "yes, but which 747 - you know, the tail number?" and they always help even if they don't know immediately. Most times I validate the information from the web, but its never been wrong. I have even had a flight attendant leave me a note with the tail number on it after I asked but fell asleep before she returned with the info. Also, if the gate agents are free for a moment before you board (don't waste their time if they are busy though), they have the information on their screens and will usually tell you which a/c too.
In over 1600 flights I have made, I have never failed to get a registration.
FWIW, on Airbus and Boeing narrow bodies either the manufacturers construction number (from which the registration can be worked out) or the registration itself is on a plate inside the foward door frame - just don't linger too long in that area or you will upset the other punters.
Andy