The first 2 questions last:
-NW never operated the DC-10-10, just the DC-10-30 and 40; NW ordered the DC-10-40s, with PW engines, directly from McD Douglas and acquired the DC-10-30s second hand from a variety of sources, many came from Swissair, NW needed the extra capicity when it greatly increased flights accross the Atlantic in conjunction with the KLM/NW alliance and it acquired rights to fly DTW-LGW (rights purchased from DL who got them from Pan Am). NW originally flew 747s accross the Atlantic on routes to some Scandanavian cities - those flights were not successful but then the KLM deal came.
-NW never operated the DC-10 into LGA, I think that the DC-10-40 is too heavy for normal ops into LGA, however, National and American opeated DC-10-10s in LGA and Eastern and TWA operated L1011-1s into LGA for years.
Your main question....visual differences between the DC-10-30 and 40, I really do not know....are the engine cowlings different between the GE engines of the -30 and the PW engines of the -40? That could be something. I do know that the landing gear is different on the -10 from the others, it is "missing" the third center set of main landing gear due to its lighter weight.