How to recognise the Tupolev Tu-154, Tu-204, and Tu-334?
First, the Russian airplane industry is organized differently from its Western counterpart. While both have research organizations and aircraft manufacturers, in Russia and Ukraine design offices are between both. In Russia, TsAGI does most basic research and develops aerodynamic designs which are then put into fully developed aircraft by the design offices (Antonov in Ukraine, Ilyushin, Tupolev, Mikoyan (MiG), Sukhoi, Yakovlev or Beriev in Russia). The finished designs are then transferred to manufacturing plants like Aviakor in Samara which churn out the actual hardware. If the demand for Tupolevs goes down, Aviakor will switch to Antonovs or Ilyushins, so the fluctuation for products of a particular design office can be evened out.

One consequence of this arrangement is that aerodynamicists have the first say on the shape of the aircraft. Russian designs generally have more refined aerodynamics, at the cost of a higher manufacturing effort.

Older airliners from the Fifties and Sixties were designed with dual use in mind, and many of them had a glazed nose, which makes them easy to distinguish from their contemporaries from the West. 

Tu-154

This design from the 1960s is similar to the Hawker-Siddeley Trident or the Boeing 727. Its production surpassed 1000 aircraft and ended only in 2013.



Tu-154 of Uzbekistan Airways (Picture source).

Four items make it easy to identify:


The landing gear uses six-wheel bogies, which is unique among three-engined airliners.
For aerodynamic refinement, heavy use of Küchemann bodies is made in the T-tail intersection and for stowing the landing gear.
The aerodynamic refinement also shows in the length of the conical section of the fuselage, which takes much longer to reach its cylindrical shape. This is a consequence of putting aerodynamics before ease of manufacturing.
The central air intake is of elongated shape, while that of the Boeing 727 is round and the one of the Trident was a heavily rounded triangle.


Tu-204

Intended to replace the Tu-154, this two-engined airliner flew first in 1989 and looks similar to a Boeing 757. It is still in production, but less than 100 have been produced.



The main differences from the Boeing 757 are:


Since the cockpit of the Boeing 757 is identical to that of the Boeing 767, it looks oversized for the narrow-body fuselage of the 757. In comparison, the cockpit windows of the Tu-204 are smaller and look more natural.
The top of the 757 fuselage slopes down over the full length of the vertical tail, whereas the downslope of the Tu-204 fuselage starts only at the rudder.
To distinguish the Tu-204 from an Airbus A320, look at the wheels (four-wheel bogey on the Tupolev, two-wheel gear on the Airbus) or the APU exhaust, which in case of the Tupolev is a small tube sticking slightly out from the fuselage, while the Airbus has a metal-colored cover at the end of the fuselage.
Another difference is the Tu-204's much longer fairing for the wing-fuselage intersection.
Also, the winglet shape is quite different between those three types.


Tu-334

The Tu-334 is a shortened Tu-204 with fuselage-mounted engines. Only two have been built, so the chances to spot it are remote. The new location of the engines allows it to use a very short landing gear, and the shortened fuselage gives it a rather stubby, unique appearance.



Similar airplanes are


Short versions of the DC-9 like the DC-9-10 or the DC-9-15; here the low bypass ratio of the DC-9 engines results in much smaller intakes and longer engine nacelles. Also, the fuselage of the DC-9 is more pointed ahead of the cockpit. The cockpit windows of the DC-9 have a separate window above the regular row of windows for better visibility in steep turns, which is missing in the Tu-334.
Comac/Avic ARJ-21, a Chinese development which is based on the DC-9 fuselage, but has a modern, Antonov-developed wing and high-bypass-ratio engines. Here the upper cockpit windows are missing, so the best way to tell the Tu-334 apart from the ARJ-21 is again the more pointed fuselage nose of the ARJ-21, the bigger fairing of the ARJ-21's wing-fuselage intersection, the lower aspect ratio of the Tu-334 vertical tail and the rear end of the fuselage, which ends with a vertical edge in case of the ARJ-21.