some very newbie questions, but yes the engines 'splitting' is thrust reverser opening.
here's some of my amateur spotting hints:
74-2 - short upper deck
74-3 - long upper deck, no winglets
74-4 - long UD, winglets.
76-2 - short and fat, but still big
76-3 - longer, than a 200. no winglets. small flap track canoes
all 73's - can see the wheels when retracted and have a large fin
73-3 - no winglets.
73-4,5,6,7,8. very hard to tell apart except from length, especially now winglets are being retro fitted.
777 - triple axle bogie. knife-edge tail (as opposed to 76 which has a cone)
DC10/MD11 - are there any DC10's still flying? MD11's have winglets.
my advice - print off some from
www.airliners.net and study them.
cheerrs