The Cessna 750 Citation X shares the front fuselage of the Citation III with a longer fuselage, a new wing, much larger engines and a redesigned empenage. Notable other differences are the long fuselage-wing fairing under the fuselage and pointier nose of the Citation X.
Cessna 750 Citation X
The first production version has standard no winglets, but they can be fitted with winglets afterwards. Therefore this is no clear recognition point. To differentiate the Citation X from the Citation X+ you have to look at the cabin windows on the right side. At the Citation X the distance between all cabin windows is the same.
Cessna 750 Citation X+
The Citation X+ has compared to the Citation X a longer fuselage, which can be observed by looking at the distance between the first two cabin windows on the right side: this is larger than between the other cabin windows. Also the Citation X+ has factory-installed winglets.
Confusion possible with
Cessna 650 Citation III
As the Citation X shares the front fuselage with the Citation III you could mistake one for the other. However, the Citation III is smaller, and has different engine nacelles and top of the vertical stabiliser (no large fairing behind the horizontal stabiliser).
Bombardier Challenger 300/350
Less likely you may mix-up a Citation X with the Challenger 300/350. However the differences are many: the Citation X has more cockpit windows, larger nacelles and a large fairing behind the horizontal stabiliser, to name a few.
Cessna 700 Citation Longitude
The Citation Longitude has relatively smaller engines than the Citation X, and four cockpit windows istead of six.