Dassault Falcon 900
Blow up a Falcon 50, put more and smaller cabin windows in it and you get a Falcon 900. This is essentially what Dassault did to come up with an answer to the Canadair Challenger with its wide cabin. Apart from being longer, the Falcon 900 has a wider fuselage compared to the Falcon 50, Dassault's original tri-jet. For the rest the external appearance is about the same.
It is nearly impossile to clearly distinguish different versions of the Falcon 900 from the outside. The cockpit could give more clues, but is decisive either.
Falcon 900, 900B, 900C & 900MSA
The Mystère-Falcon 900 family started with the Falcon 900, succeeded by the 900B. The difference between the Falcon 900 and 900B are the engines: TFE731-5AR for the Falcon 900 and TFE731-5BR for the Falcon 900B. They fit in the same engine nacelles, so there is no external difference.
The Falcon 900C retains the engines of the 900B, but has cockpit avionics of the Falcon 900EX, so you have to enter the cockpit to see this.
Falcon 900MSA is the designation of the Falcon 900s used by the Japan Coast Guard for maritime surveillance.
Falcon 900DX, Falcon 900EX & Falcon 900EX-EASy
The Falcon 900EX is a Falcon 900 with yet different engines (three TFE731-60s), other systems/avionics and an improved performance. But there are no external changes. The same applies to the version with the EASy (Enhanced Avionics System) cockpit, marketed as Falcon 900EX-EASy.
The Falcon 900DX is the same as the Falcon 900EX but with reduced range and therefore a lower price tag.
The Italian air force uses both the Falcon 900EX and 900EX-EASY. These are designated VC-900A and VC-900B respectively.
A Falcon 900EX-EASy fitted with winglets is called Falcon 900LX by Dassault. At the moment this is the standard version. But winglets are no strong recognition point as older aircraft (even early model Falcon 900s) can be equipped with winglets as well. Then they do not receive the 900LX designation.
The Royal Air Force designates this aircraft as Envoy IV CC Mk 1 (Envoy IV CC1 in short).
Confusion possible with
Given that the Falcon 7X and 8X are derived from it you are most likely to confuse them with the Falcon 900. Look for the differences in cockpit windows and main landing gear doors, and the saw tooth versus straight trailing edge of the vertical stabiliser.
The Falcon 50 is the predecessor of the Falcon 900. It has a narrower fuselage with less, but bigger cabin windows. For the rest the aircraft are much the same in appearance.
The three-engined Falcon 900 shared a lot of components with the Falcon 2000, so if you see only a part of the aircraft you might them mix-up. Otherwise the third engine on the Falcon 900 is a very clear recognition point!