MD-11 vs DC-10 (and MD-10)
MD-11: Winglets - they all have winglets
MD-10, DC-10: none have winglets
MD-11: Main Landing gear - - Left, Right, and CENTER
MD-10-30: same
DC-10-30: same
MD-10-10: NO CENTER GEAR
DC-10-10: NO CENTER GEAR
(-20, -40 ?? I don't know)
MD-10 vs DC-10
From a long distance, it's impossible to tell.
When you get close enough, the first thing you can see is:
MD-10: UHF antenna situated above wing (same as MD-11)
UHF Antenna MD-10
DC-10: UHF antenna situated above main cargo door (of freighter version, of course), just aft of nose gear
UHF Antenna DC-10
Still closer from the right front:
MD-10: No flight engineer panel visible through right-most window
See-thru window 
DC-10: Flight engineer panel blocks right-most window
Blocked Window
And from the left side:
MD-10: Outflow valve (on left side of fuselage behind main cargo door) is a single door, square, identical in appearance to MD-11
Outflow Valve (look under the "Dash" of MD-10)
Outflow Valve (look under the "X" in the FedEx logo on the fuselage)
DC-10: 3 pieces - ram shield, outflow valve, and thrust recovery valve - make up the "out flow valve compliment" (visible on the ground)
Under the "X", left to right: Ram shield, Outflow valve, Thrust recovery valve
Still closer:
MD-10: MD-10 painted under the clearview window on both sides of the nose
"MD-10" painted on both sides of nose
DC-10: nothing painted in same space
Slick nose
In general:
MD-10: found in the better domestic layovers carrying happy pilots
DC-10: found in the ratholes of the US carrying grumpy captains, lazy F/Os, and either a happy young lad or lass eager to upgrade, or an ancient aviation relic killing himself to improve the retirement he'll enjoy for only a few months beofre he keels over
(just KIDDDING, guys - - relax !)
And finally, the easiest way is to watch the guy doing the walkaround - - if he walks around the main gear clockwise, it's a MD-10. If he walks around the main gear COUNTER-clockwise, it's a DC-10.
Now that I think about it... if the second officer does the Exterior Safety Check, it's a DC-10, and you'll see him walk to the tail to look up at the #2 engine and APU Inlet/Exhaust area. If the Captain does the check, it's an MD-10, and he'll view the tail from the crew stairs.
(I think I could have turned that into a Top Ten list.

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