The Citation Excel series is essentially a crossbreed of the Model 750 Citation X and the Model 560 Citation Encore. The former supplied the fuselage, which was shortened. The wings and tail are of the Citation Encore.
The Citation III was the first Citation with a swept wing and a T-tail. It can be distinguished from similar bizjets by its cockpit windows, squared off oval cabin windows, large thrust reverser fairings and bullet fairing on top of the vertical stabiliser.
The Citation Sovereign can be regarded as a stretched Citation Excel. However, it is a lot bigger and also the tail is significantly different: it looks a like that of a Lockheed Jetstar.
The Citation Latitude is a completely new design, although some elements in the external appearance look familiar. It looks most like a shortened Citation Sovereign, with bent-up wingtips and the delta fins of the Citation Excel. The cockpit windows are also different, and not looking like the ones originally developed for the Citation III anymore.
Cessna 700 Citation Longitude
Externally the Citation Longitude is very similar to the Hawker 4000. You can best keep them apart by looking at the engine nacelles and winglets. The Hawker 4000 lacks winglets and moreover has large external fairings for the thrust reversers.
Being at one time the fastest civil aircraft on earth its sleek lines are recognisable. It looks like an enlarged Citation III with huge engines and long, slender wings.
This is the original Cessna Citation series, having straight wings, engines at the rear of the fuselage and horizontal tail plane attached to the lower part of vertical tail plane. The horizontal stabilizer has a V shape, with a noticable dihedral. The cabin windows have a characteristic Citation shape: a vertical ellipse, with the top and bottom cut off. Also note the relatively large cockpit windows.
Take the fuselage and the wings of the Citation I, put a T-tail at the back and different engines, and voilà, basically you have a CitationJet, the first and smallest member of the CitationJet/Citation CJ series. So if you know the Citation I, the CitationJet is not so difficult to recognise. Other members of the Citation CJ family mainly have a longer fuselage, but also other changes compared to the original CitationJet.
The Cessna Denali very much looks like the PC-12. The air intake and nose gear of both aircraft are a bit different, but it is easier to look at the tail area. The Denali has a smaller dorsal fin than the PC-12, no ventral fins and no bullet fairing in front of the horizontal stabiliser. (photo Textron Aviation Inc.)
This looks like a scaled down Challenger 600 series, with engine nacelles with a single exhaust. A distinctive feature of the Challenger 300 and 350 is that the cockpit side windows become narrower towards the end.