The workhorse for the domestic Aeroflot network was a trijet, like its western counterpart, the Boeing 727. The Tu-154, NATO code name Careless, has three engines at the back of the fuselage, two attached to the side and one in the rear fuselage, being fed by an S‑shaped duct with the air intake in front of the vertical stabiliser. Furthermore the Tu-154 has the typical Tupolev main lan­ding gear which is retracted rearward in pods extending from the trailing edge of the wings. Both main landing gears have six wheel bogeys. The nose gear retracts rearward as well. Finally the Tu-154 is characterised by a sharp forward pointing spike or bullet fairing on top of the vertical stabiliser.

The different versions of the Tupolev 154 can be distinguished by

• the shape of the nacelles
• the shape of the middle exhaust
• the shape of the APU exhaust
• the presence and shape of the bullet fairing on top of the vertical stabiliser 
• the number of emergency exits
• the width of the ailerons
• the presence of a cargo door in the left front fuselage
• the presence of tubes at the rear right side of the fuselage, and on top of the middle air intake
• the presence of a vertical protrusion on top of the vertical stabiliser

The first variant of the Tu-154, without any suffix in the designation, is especially recognised by the forward pointing slender spike at the top of the vertical stabiliser. While it has an APU at the base of the vertical fin, the exhaust is not clearly visible like on later models. The Tu‑154 has two normal sized cabin doors in front of the wings, and two overwing emergency exits. The aircraft is powered by three Kuznetsov NK-8 engines, which have a single exhaust. The outer engines have typical exhaust for the thrust reversers at the top and bottom of the nacelle.

In the improved version, the narrow spike on the tail has been replaced by a thicker one. Also, the APU now has a clearly visible exhaust pipe, above the middle engine.

The Tupolev Tu-154A and later versions have a thicker spine or bullet fairing pointing forward from the vertical fin. It retains the two normal cabin doors and two overwing emergency exits of the Tu-154. (photo: Eduard Marmet/WikiMedia)  

Internally, the Tu-154B is quite different from the previous versions, as the wing structure is beefed up. However, you cannot see this on the outside. You can however see the additional emergency exits, just in front of the outer engine nacelles.

The Tu-154B-1 and Tu-154B-2 only differ from the 'straight' B on the inside, like systems and interior.

The Tu-154M can be regarded as the second generation Tu-154. The main difference is in the engines, as these are now Soloviev ones. The outer engines have clamshell or bucket type thrust reversers, with external fairings. The shape of the middle engine has changed, not in the least because the APU exhaust above has gone.

Several Tu-154s were converted to cargo aircraft, by adding a large cargo door in the left front fuselage. Most cabin windows remained however, so this Tu-154S is less easy to recognise than most western passenger-to-cargo conversions. To make room for the cargo door some cabin windows have disappeared, breaking the normal continuous row between the main cabin doors.

You cannot clearly see the large cargo door in this Tu-154S, but the the interruption of the row of cabin windows between the two main cabin doors is another sign. (photo: Alan Durand/WikiMedia)

Tu-155 is the designation of an experimental version of the Tu-154, to test the use of alter­native fuels. These fuels were liquid hydrogen and liquified natural gas (lng), stored in a tank in the rear of the cabin. External pipes along the side of the fuselage were used to get this fuel to the engine. The area with the fuel tank in the cabin was ventilated to a protrusion on top of the vertical fins, in place of the normal forward pointing spike.

The Tu-155 was an experimental version of the Tu-154, to test the use of alternative fuels like liquid hydrogen or natural gas. The tank for this is pleced in the rear of the cabin, hence there are no cabin windows there. (photo: Aeroprints/WikiMedia)

Here you can see the fuel pipes on the outside of the fuselage. Alsno note the protrusion on top of the vertical stabiliser, for a ventilation system, and the absence of a forward pointing spike. (photo: Juergen Schiffmann/WikiMedia)